Summit's catalog.

https://www.summitracing.com/parts/nal-25161284

I wouldn't say that it's the Bible, but just a lot of info about the shortcomings of the stock system. The part# that you're looking for is #25161284. I got mine from Amazon and put it directly in the hands of my mechanic. Even though he was referred to me as the best wrench in the area he knew nothing about that valve.

BajaRooster: I make sure the center valve is removed (because I won't use the side tap ports next to the boss) and put the known 30# valve in the offset port, yes? Not sure I understand how
a plug in the center position, enables oil to return to the block����

Replacing the oil cooler.
 

I'm settling on the 30# bypass for the outgoing port to the filter & removing the bypass altogether in the center return from the filter. Since this is the 3rd rebuild in what I can see the last 70 hours I will replace the oil/PS cooler which is 2" X 17" & looks to be a Merc 806423T (or aftermarket copy). Is there a pretty simple swap/upgrade that gives me a bit more capacity/insurance???

I would worry about upgrading the oil cooler until you know your oil Temps. Buy the original size cooler and install it as well as an oil temp gauge. This way if you are getting to high of oil Temps you can look into it at that point.

I pulled the valve out from behind the center nipple as it's unnecessary and just another restriction, but it isn't a big deal to leave it in there. The new valve replaces the one in the offset port.

You can upsize your oil cooler but get one that's thermostatically controlled and you'll be fine. I went a little big on mine and have to get a thermostat as I get a lot of condensation in my oil at the moment.

Link to be sure my builder is on the same page as you guys...
 

I'll be picking up my 502 long block to bring it to a reputable circle track race engine builder...I just want to go over the important requirements of the marine engine environment, etc. would love to have a discussion punch list for him...trust but verify.

4100# 24' cobalt with Bravo One 1.50 drive & 19p Mirage Plus prop (for the moment)...it was dyno'd @527/570 in '11. Thunderbolt IV ignition.

I will at a minimum replace the stock oil/ps cooler with a new clean one from MrCool...then monitor oil temps...

Remember this will be the 3rd rebuild of the Gen VI GM Crate Motor model 12568778 from '06...the heads are GM marine
heads #12562934 (not sure which rebuild)...Only 70 hours in 10 years.

The last rebuild (2011) upgraded the crank, cam, lifters. Has forged pistons. It has EMI Thunder exhaust which routes through the drive (corsair through stern has been abandoned/disconnected). Has Edelbrock 7562 Performer RPM Air-Gap intake... Has Edelbrock 1410 Performer 750cfm marine carb...
I already bought the 30# oil filter by-pass . I will upgrade the rockers and springs (recommendations?). I'll reassess after the tear down and discussion with the builder.

So regarding piston to cylinder wall clearances, marine head gaskets, etc. what should I bring up or remind him. Seeking words of wisdom from this awesome site.

Find a reputable Marine Engine builder. You should not have to be giving "tips" to the proper shop.
A 500+ HP engine should not be running through prop exhaust.
You need to do more research before letting anyone work on the engine or your next thread will be about the 4th rebuild in 80 hours.
The oil system changes you are talking about are only two of many important engine requirements.

Nor a 750 Edelbrock.

If there is a better source to do my "research" let me know, please. I thought this site would be considered research, in a way...The shop I have entrusted to make this motor right is capable (Google: Earl Wham Jeff Bird) as the best builder in Northwest if not the country...Pretty sure if I give him some specs, he can make it happen.

The last "builder" of the motor was supposed to be "the sh*t" near Seattle but is now out of business...Maybe they got so big the right people weren't doing the work? Who knows. All I'm asking from you guys is some bullet talking points to go over with him (he may already know)...He mentioned even toning down the horsepower if needed by replacing the cam and get back to a more reliable build...If I need to re-plumb the Corsa Exhaust that's what I'll do. The parts are in a box & need some parts & new harness, I'm told. If I need to get a different carb, I'll do that, too. We'll know more when he tears it down. He mentioned signs of water around the exhaust port of #3 (the problem cylinder) from the starboard head...He just has 4-5 builds to wrap up before mine. Also the bellows had water in it when the drive was pulled.

I appreciate the help, I'd just like to have some knowledge going into it. Thanks.