The question from above remains though. What increase of compression could be expected by changing from open chamber heads to closed chamber? Also, I agree the aluminum aftermarket heads would be ideal, or tear the short block apart for better pistons. If using the 516s, new valves, surface, hardened seats, etc it doesn't seem like a cost effective way to go nowadays. Milling the heads was quoted at $150. Yes 0.0123" taken off the intake side for every 0.010" taken at the flame face. The heads are 452, valves are good and they are factory with hardened seats. Seems like the way to go in this situation.