Whatever you do you need a 78 cc combustion chamber with iron heads,less with aluminum, or it will be a real dog. A set of 440 source heads milled about .040, and a thin head gasket might work, or buy the aerohead 516s already done. Make your life easy and fix your heads. A 452 would drop compression a bunch, milling them and an intake to fit not worth it in my opinion, plus the pushrods are to long after milling, more money.