Stainless Marine for sure!
Stainless Marine Exhaust for SBC :: STAINLESS MARINE :: Exhaust Systems :: Diamond Performance Parts

What makes Stainless Marine better than any other sbc exhaust company???

Eddie Marine is another source
https://www.eddiemarine.com/shop/emi...ll-block-chevy

IMCO is another exhaust manifold. Have them On mine. No problems

IMCO Powerflow will work. Thumper won't since they are wet joint. Hardin's are same thing as IMCO's and $500 less per set. Stainless is just too much from the cost to performance perspective. From the research I've done, the gains just aren't there on SBC's.

20-30 hp if tuned properly???
That's less than 10% gain...

I got stock 350 Mag MPIs. Not much tuning I can do unless I reflash ECM which I'm not doing. They are what they are pretty much. Only reason I'm not going back to stock manifolds and risers is that I want the weigh reduction. Lot of money to be spending just on weight reduction, I know, which is why the stainless marines are just out of the question.

Stainless marine will last the longest. EMI is my best seller because of the price, IMCO uses a gasket between the manifold and riser, (wet joint) EMI & SM use a jumper hose for a dry joint.

If your in fresh water there should not be much if any of an issue. Installed EMI in 2015 during repower, manifolds are from about 2011 (I think) and I'm in fresh no issues.

For that matter, I still have my original 1994 BBC cast iron and they look fine in the back of the shop, need to toss them