I'll jump on the telling you what you should do instead of answering the question ... With open chamber IRON heads you don't really want to go much higher than your are now with your compression ratio due to the risk of detonation. Your heads have already had 8-10 cc cut out of them , I'm not sure what that .075 measurement is from what to what but if it is piston to head you can close it up to .035 -.040. What bore size ? You want to make sure the edge of the chamber is not overhanging into the cylinders .