If the pistons are way down the hole i wouldn't think you would notice any diff with .020 vs .038's on a open chamber head. Now if thats your case and your so inclined, look for some 516 close chamber heads swap all the valves and springs have the 516's cut for the 1.74 exhaust, do a little pocket porting, and have them cut .005 to make sure their straight and use the steel gasket, you would be betwen a .5 to .75 gain in cr and the 516 will still flow good for up to 325/350hp