You will be able to spin it an easy 6K. Assume they are open chamber heads, which will put them at 68 -72 cc"s. To reduce the combustion chamber volume by 1 cc. cut .0048 off the heads and for every 10cc's you mill the heads, remove .0063 off the intake gasket sides. If you don't want to CC the heads ( and you should ), go with 70 cc's.