what fuel octane you planning on running? 350 Chevy comp ratio's didn't get reasonable until 1972, and they ran with 64cc heads, I don't want to know what 58cc heads will do, I don't have any real numbers to go off of but 12.5:1 wouldn't be out of the question, well over the limit on 93 octane pump gas. 305 heads choke 350's anyway.