1980 470 Mercruiser
 

Can I replace my cast iron exhaust manifold and riser with the later aluminum pieces and what parts do I need to match up to the exhaust without replacing the inner transom plate?

you can replace it, but unles you find a good used dead engine to get the parts off of,its going to be expensive$$$$$$. personally, I'd prefer the cast iron manifold, they gave a lot less trouble. I think your engine has the 3" exchanger, you would have to upgrade to teh 4' if you run the alumium manifold(aluminum transfers more heat)

Look on ebay, theres manifolds and risers on there for sale now. Just look under mercruiser 470 or 3.7 or 224. You stand to save alot of money!

They also got some heat exchangers for sale on ebay!