Two details. One-shoulda been mentioned right off the bat-exhaust. What are you going to be running? Stock merc center dumps? Limits cam. I think 731 too hot. 454 HO roller cam about your most radical with stock exhausts. Two...where are you? I may want the old dead 330 engine if you are on east coast.

Im looking to find out which rocker stud kit I need and now see they come in different lengths. What "effective stud length" will I need? 1.520? 1.750? 1.900?
My RHS heads have non adjustable type rocker bolts as per typical Gen VI configuration.
I will be buying the COMP Cams Ultra Pro Magnum Rocker Arms 1620-16.

Stellings Dry system for exhaust. Not headers. Aluminum manifolds.
Im on Long Island and the engine will be available in about 3 weeks. Its still in the boat.
Im pretty sure its a gen IV

How about a custom cam from Bob M?

Absophuckinglutely!!!! people have been modifying merc 454s for what 25-30 years now, and in that time nobody has found a common off the shelf cam that works worth a damn. These are Chevy bigblocks, the technology is insane. Rocket scientists look at boat motors in awe.

I don't know about the RHS heads, but these are the studs you need for GM 088 rect port heads to convert from non adjustable to an adjustable valve train.
https://www.summitracing.com/parts/a...7102/overview/

Remember, Merc made a 7.4L HP450, which used identical parts (other than short block) that the 502cid HP500 used.
I believe the HP450 used same shortblock as the 454 Mag's.

Your asking good questions , and trying to get clarity on the unanswered questions.
Have never heard of a merc spec forged flat top. And it sounds like your rotating assy is balanced for the 9cc mini domes the rect port head uses. You will have to confirm the grams of the gen 6 piston that was supposed to be with the forged crank 454 mag combo and buy another set if you dont have the pistons that are supposed to match the 119cc chambers if thats what you have .
A good budget cam to consider with your exhaust is the zz502 cam .will give a nice rumpity idle and a few more ponies over the mag cam without needing real stout valvetrain pieces.
Please dont put 119cc heads on a block with flat tops and upgraded exhaust . Please.

Please explain. "Please dont put 119cc heads on a block with flat tops and upgraded exhaust"
Flat tops no good? Compression too low? Bad quench area?
Also if given the choice between the ZZ502 and the Crane 731 which one suits my application better? I like that the ZZ502 cam is $210 instead of $425.
Im about to order stuff.
As for the pistons the have one fly cut upper and one fly cut lower. So two fly cuts overall. Does that help clarify? I can take a picture if needed.

Do you have any pictures of these RHS heads that are 320cc, have a 2.19 intake, and came on a reman from mercury?

I know that engine quest makes a gen 6 style replacement core, that is a direct replacement for the GM/Merc head. It is totally different than the 320cc RHS head, that guys use for making big power. The 320 RHS/engine quest head, does not have the center heat riser passage, has raised exhaust ports .500, 7/16 rocker studs, rocker bosses shaped to fit shaft style rockers, and come with 2.25 intake valves