smugglers blues

Hey everybody, I need help. I have an 84' Scarab III with twin 454 mercs that are pretty much stock. I have had engine trouble/exhaust chaos so I have been replacing the exhaust system piece by piece. I have all new manifolds (GLM) which I now learn on the net that might have been a mistake but its done. I ordered new 3 inch spacer blocks (Mercruiser) and I haven't ordered riser/elbows yet because I am puzzled. I couldnt figure out why everything had so much corrosion but the elbows looked almost new. I have had the boat for 8 years and who know how long this stuff was on there before I got it. Yet these risers look great. Then I realized they were stainless, I thought this must be a good thing! Then I start getting mixed reviews, some say stainless are the way to go and if you don't see corrosion they are fine. Others tell me the stainless elbows are junk, Mercruiser stopped making them because they wouldn't hold a seal at the gasket. Some say it was mounting design, others say its heat expansion rate difference between the steel and the iron....do I buy new iron elbows? Keep my stainless? At 8 yrs old are the stainless probably shot anyways even though they look fine? Please help!