Re: 302 or 351
Depends what you want to spend. HP=$$$ The more you spend the more you get. How much are you spending? 2grand or $3600? $2000 is barely enough for a decent rebuild. You'll still get 300hp from a 302 fairly easily. You'll just have to go a little bigger on the cam, but again with a mild port job on the heads and a balance.
A complete cam change would be around $600, You'll need cam, lifters, valve springs to suit cam, manifold gasket, new timing chain, timing chain gasket, water pump gasket and head gaskets as the heads will be taken off for the P&P
What about the bottom end? new pistons, bearing, balancing? You'll soon be over 2grand