exhaust manifold
#3
If it's a completely stock 502, probably not much if any performance gain going to a performance exhaust or header. Stock Merc manifolds and risers are plentiful and relatively cheap if you want to keep costs down, or look for a set of the Merc 496 manifolds and risers. There is an aluminum version of these that will save some weight, and they also eliminate the wet joint between manifold and riser that is the demise of many engines when they start leaking. Other than that, take your choice of IMCO, Stainless Marine, Gil, etc.