If you're going with changing the pistons the. You don't need to mill the heads. You will need to ports them and maybe look at going to bigger valves. With all that said and the money spent on first buying the 302 or 308 heads then rebuild them port upgrade to bigger valves, you'd be better off buy a set of aftermarket heads. Based off you goal and to keep cost down, go with a set of RHS heads and get the version with the 2.02 intake 1.60 exhaust.
