I want to build my merc 500hps to make 650 hp what parts are good quailty to buy to get me there?

I know someone that just did this and ended up with 650hp and 660hp on the Dyno. He stroked the motors to 540, changed cam, went with aluminum heads and headers. Should be nice reliable NA powerplants.

Worked heads-intakes-carbs, cams, maybe up the compression a lil. Not too hard to do. If you have the loot you can stroke them.