would I be wasting my money putting emi thunders on a stock 502?
 

and running them through the prop?

yep.

are you sure you dont need to think about that? lol what if I had captains choice but motor was still stock?

Going from thru prop to thru hull with Capt Choice might add a couple hundred rpms at WOT.

Going thru the prop restricts both banks down to an opening that is maybe 3 1/2 by 2 1/2 inches. Kind of like running your big block Chevelle's exhaust into a y-pipe and a single tailpipe. No real gain will be seen even if you added free flowing headers because the main restriction is downstream. And on top of that, even if you had thru-hull exhaust, adding aftermarket performance manifolds to an otherwise stock 502 with its low lift cam is not going to give you very much performance wise. These would help more if you were running a bigger cam, higher compression, and better flowing heads.

Now if you were running in salt and were switching to something like a set of Stainless Marine manifolds, I would say you might get a benefit in longevity by eliminating the wet joint between the riser and the manifold. Corrosion and leaks in this area has killed many engines.

Thanks for the advice as always guys!

there is one advantage to aftermarket exhausts. when she bends over the back seat to check out your engine...