Well said! Bare Promaxx 290cc would be my choice if new heads are needed. Those promaxx heads alone would make atleast 30 more hp.

From the photos when I searched for your heads...they appeared to be not as large (the port opening at intake joint) as a true Rec Port. As an example, the 088 heads have 30 more CCs on the Int runner.

088 heads in good shape are not that cheap, you.may get lucky. Do a valve job and guide work...maybe some new springs... for your specific cam etc. You're into it for almost the cost of new bare heads.

Your heads should be gtg with some work. But will likely need some new int valves. Not worth the labor to try and get them straight to less than a thou of runout and then trust them.

Sure looks like a crack around 1 o'clock on the exhaust valve in the first picture.

From looking at all the pics I'd have them resurfaced. You're not going to get near the valve seat with the little bit that is going to need taken off. As for the valves, put a good set of valves in it and be good to go. Remember that marine valves are not like car or even race valves. They need to be at least a Heavy Duty or Inconel. Have a good 3 angle valve job and back cut the intake valves.

While the heads are apart you may as well clean up the bowls if that hasn't been done already. I've seen 30-40hp just from that alone.

I thought this was an interesting thread and I have a slightly different yet similar situation with my GM Rectangle port cast iron heads too ---- trying to squeeze as much power out of them as I can without having to buy/upgrade to better heads and perhaps the Original Poster and others might find kinda interesting and be able to learn/glean from some input and ideas here.

I currently have some GM Rectangle port cast iron heads as like the OP is possibly purchasing if can find some used ones in good acceptable condition. They're on a pair of my Naturally Aspirated BBC 482cid (4.25" Stroke X 4.25" Bore) engines that were Dyno tested ---- 510hp @5000rpm and about 547-lbs of Torque at about 4000rpm or so.

I recently installed the engines in the boat last year--- one of more positive things is that the boat is equipped with Stellings full length tubular dry headers for any power that I can squeeze out of my slightly too mild 482cid engines.

I believe the Cams are a little too short and probably should have used a dual plane air-gap intake manifold on the engines as I also believe they might act a little soggy with the Single plane intake manifolds I have on the engines now. I had the cylinder bores "notched" so as to unshroud the intake and exhaust valves on such small diameter bore size and try to give it a little more power.

I believe the bore notching lowered a few 10ths of Compression down to maybe even 8.6:1 comp ratio.--- kinda low, but perhaps ideal for a supercharger in the future. (?)
Anyway, I had the GM Rectangle port cast iron heads bowl/pocket ported ----- just some blending and clean up, but can't remember if I had anything done to the combustion chamber side to blend any ledges/ridges.

And I don't think we did any grinding of the valve guides to narrow them down, but I'd like to do that to reduce any airflow obstruction in the ports...

...but I wonder if it would help any on a BBC only making peak power @5000rpm (?) 🤔.... especially if/while I am only cruising somewhere around 3500-4000rpm in a heavy 40' boat. But if the cost of working over the valve guides is inexpensive enough, then perhaps I may have it done even if it only yields small power gains.

I'm a nerd for efficiency. Besides, those Valve Guides in those GM heads look like they're mini Helo-Pad Landing Zones just begging for some little mods.

My 482's were freshly rebuilt a few years ago and sitting on the floor with most all new parts except for the GM Rectangle heads, carburetors and the distributors.

Cams are Hydraulic Rollers and I don't want to change those even though they're a bit too small peaking only @5000rpm.

Articfriends/Smitty suggested replacing my GM Rectangle heads for some new Aluminum ProMaxx 290cc heads with their 110cc combustion chamber in bringing up my current Comp Ratio up a little more which would help give a little more kick from the compression alone besides a better breathing head which sounds all very tempting but I want to try and stay with the GM cast iron heads that I already have on the engines however may be willing to take them off for some further porting work and maybe see if I can use a slightly thinner head gasket to help bring back some comp ratio.

One thing I may do for sure is swap out the Single plane intake manifolds for the dual plane air-gap intakes.

Chime in if you guys have anything to add or suggest with the old GM Rectangle port cast iron heads design.

Kaama im really looking at those promaxx heads. Under 1500$ bare is actually a pretty good price. Then I pick my valves and hopefully my springs work with a cam choice. Maybe some minor porting. I'm trying to make a reliable 500 hp on an 89 octane motor. So I need to stay under 10.1. Does anyone know what exhaust manifolds would work best with those promax roval port heads? I was probably just going to go with a gil/hp500 style

Summer, yes I believe a GIL exhaust manifold and DRY pipe system would be fine enough or even a Stainless Marine exhaust system might be a hair bit better but you want the tailpipes to be DRY as a bone ---- don't 🚫 let the exhaust gases mix with the water in the tailpipe. Just my .02

I just found out today when it all adds up, that by the time I spend the money 💰 on doing mods on the GM cast iron heads that I already have, I just might be better off buying the dang ProMaxx 290cc heads instead and still be way farther ahead. 🧐 🤔

Kaama, I had factory manifolds and risers. And the cam that was in there was a 236 244 .5 overlap.....probably too close for comfort. You say dry tail pipes you mean dry joints and the water mixes at the back of the pipe right? I'm trying to keep my mufflers on as well so I need stubby risers. Are the stainless marine a bit better then the gils?
The 290 numbers seem to be impressive for the buck. It makes it hard not to buy them . I'll probably have close to that much trying to save my heads. I'm unsure of the cc of my heads as they look to appear have been worked

Oh dude, you just opened up a can of worms with your question....haaa 🤣😉
Just so you know, I am NOT an expert, but I have some experience with this Marine hi-perf stuff since the early 1980's. I have seen a lot of trial and error while growing up and living in a small town on Lake Michigan.

Back then several of my friends were experimenting with engine upgrades and mods, etc and that also included aftermarket marine exhaust systems for better breathing capability for more power.

That being said, anyone can jump in here and correct me if I am wrong about anything and/or add their knowledge to help you or anyone else out here, ---- but I will tell you what I have witnessed and what I know based on what I have experienced and gleaned over the years.

A dry exhaust system aids in producing more power by not having to push the obstructing water out of the tailpipe usually referred as back pressure.

A dry exhaust system will also prevent "reversion" of water 💦 being sucked back up into the engine which can have damaging effects.

Another benefit of an aftermarket Marine performance exhaust system is that it is usually made out of cast aluminum which lightens the weight up a bunch compared to a HEAVY cast iron, very crude breathing, bulky clumsy exhaust system... especially much the way they came on MerCruiser engine packages in earlier years that guys like me were dealing with way back then.

There too many variations of aftermarket Marine performance exhaust systems and how you can set them up with a given "riser" or tailpipe configuration and how/where at what point you introduce the water 💦 into the tailpipe, etc and begin to mix it with the exhaust gases.

However, you should be good to go with an exhaust system where the water mixes in with the exhaust gases at the very END of the tailpipe... much like the way Custom Marine does where the 2 walls of the exhaust pipes are basically swagged together with some gaps for the cooling water to escape.

Back when GIL Marine exhaust systems were available, they usually came bone dry with water bungs welded into the tailpipe so that when the tailpipes were installed in the boat the bungs were just on engine bay side of the transom and then having to connect a water dump hose to the bung and then drill a hole for each tailpipe through the transom/hull and install a stainless steel water dump. GIL Marine would completely weld the 2 tailpipes together at the very end and drill about a 1/8" very small drain hole at bottom of the pipe through the welded seam so the pipes would drain properly when the engines were not in use.

GIL Marine also made their tailpipes another way where the cooling water was introduced inside the pipe and further UP the pipe for a specific separate thru-hull exhaust system using a big rubber hose to connect the actual tailpipe (which they began to refer to as the "riser") which connected to thru-hull exhaust piece via the big rubber exhaust hose ---- what a bunch of BS in my opinion.

But I think they mainly did this to help reduce the exhaust noise of the engines as these kinds of boats were gaining in popularity as the years went on. And then came all the muffler systems and switchable exhaust systems BS, etc etc etc. 😐🤨
And all extra BS exhaust stuff did was KILL POWER ‼️

Stainless Marine exhaust systems I believe are still in business and I am quite sure I have seen different configurations of their tailpipes --- bone dry, wet, and semi wet, etc. And their are a few other manufacturers out there too, but do your due diligence and find/choose a good breathing performer.

One of the things that I did NOT like about the GIL exhaust manifold if I am remembering correctly is that if it was necessary to get your hand and fingers in there when it came to changing out the spark plugs you almost needed another thumb on your hand right about where your hand and wrist connect together made it a pain in the arss to swap out spark plugs. That was back around 1983 working on a friend's boat.

Anyway, the intent and moral of the story (just my .02) is:: ---- whether you get an exhaust system from Stainless Marine, GIL, IMCO Power Flow Plus, Dana, or whatever, choose to make it a DRY system as much as you can stand to do.

The most water that is introduced into the tailpipe the more you're going to kill the power, torque and RPM.
If you feel the need that you have to add mufflers to the tailpipe then put a muffler on that is not smaller than the diameter of the tailpipe itself... and I understand that you need water dumping on the mufflers to keep them cool in some way.....but anything that impedes the exhaust gases from flowing and breathing well is kinda like shooting yourself in the foot when it comes down to any kind of performance. 🧐

I'm going to go get a another cup of coffee ☕ now 😌