for sure a 600-650 .. depending on your cam and gears. U didnt say what rpm range you inttend to make your power in. there is a BIG difference between an idle to 5500 motor and a 3500-7500 rpm 306. For a street car I would say 600 ideal 650 max for race purposes I could see a 750 on a good breathing high rpm motor. Really its a shot in the dar with out you providing everyone with complete details ... cid , cam , converter stall speed or running a manual trans , rear end gears and tire height. with that info u would get a much more accurate guess from members then just cid and some set of heads