I use 4.370" for my 454's
17046 or 1037's

If you have aftermarket heads, be aware you may have to use a bigger bore headgasket than you’d think.

I have a pair of 1996 454s that I had sitting around after I built fresh power. We bore notched, bigger cams, swapped to AFR aluminum heads that also added a lil compression, aluminum intakes. Seen this done this to several 454s and it's tough to beat the bang for the buck. You will need AFR for a day to add the required fuel but it's not that big of a deal, and not hard to hit the 500HP with nice torque as well. Much more than that you'd be pulling it all apart to change crank so then the sky is the limit... just get the cash ready. And obviously not running stock exhaust.. Stainless marine or many other options are available usually find stuff used for the 5-600HP quite reasonable.

It appears that you have the old Merc 454cid/330hp engines with the 2-bolt main block and "Peanut Port" Oval port design.
And yes, you can do a port match when mating the head Intake runners to the Intake Manifold runners. The gaskets you're going to use may have some effect on this and don't let any silicone sealant squish it's way into the ports as the intake manifold is being installed on the heads as this can disrupt/disturb critical airflow effect power... not much, but it still will.
As long as you got the heads off I would also do some slight bowl porting work and short side radius work and blend it in nicely with the valve job

I had prematurely sent my previous post by accident before I was finished and this post is to finish what I was saying...

Add an accurate 3-angle or 5-angle valve job. Any porting should only be minor in the bowl area and around the short side radiuses under the valve---have it professionally done by someone who knows what they are doing----(not sure maybe $300/pair for what you need)----- and needs a good accurate valve job and also "Lap-In" the valves for a good tight seal and the head porter should blend it all in with the valve job.

Something else you could do as an option is to have the cylinder bores all "bore notched" to unshroud the intake and exhaust valves for better breathing around the valves. This is usually good for about 20 to 25 horsepower or so. However you need to do the math with your pistons because it will lower your compression. If you already have your Pistons with the compression ratio that you want then it may be too late to do the bore notching procedure---- But if the compression ratio and math works out in your favor and you are able to proceed with the bore notching procedure, then make absolutely sure you have somebody do it that absolutely KNOWS what the hay they're doing‼️

I would suggest you replace the valve guides with a bronze wall valve guide liner or bronze guides and make sure the guides are properly honed for the right clearances and tolerances for an engine destined for marine use.
I hope this helps you out--- have fun!

Thanks for the reply! Sure helpful

Rectangular port mag motor 4 bolt main Inconel valves etc! 330 maybe peanut round heads not as easy to soup up.

You have a good point, but he's on a pretty tight budget so I think he is wanting to stick with the simple stuff upgrades/mods. There's a couple of simple things he can do that I have mentioned to be able to give his Merc 330hp engine a little more kick to possibly a 350-400hp engine.

Anyone have a service manual for mark iv 454s? Looking for torque specs