I'll agree with Scott. Nothing wrong with the 305. As far as the heads go, they can be made to flow, you'll see alot of dirt racers use them. One thing to keep on mind about the 305 (or any SBC displacement other than 350) is rebuild cost. A 350 is cheaper to rebuild than a 305, and will make more power. Now, I'm not talking huge $$ here, maybe only about $40-50 difference, but a 350 will make more power for less $$.
