If they will fit on a 502...If you can aford it I say go for it. If you wish to upgrade later on you can always have them ported. You can run Higher CR on AL heads vs cast iron and still run pump gas. Lets not forget about the 70+ lbs you just removed from the back of your boat. I am not a Chevy guy so I really can't give you a hp #, but 500 ish should be np( I am sure you could make more). I know they will "bolt up" to the exaust. I do not know If they have a higher/ lower postion then the stock( once again not a chevy guy)
GL