Dennis:
I made some rough calculations that have some variables in them, so the results are only indications of what will happen when you use different heads. I didn't calculate the CCs of the 239 heads, since I don't have any figures for other heads either.
When you took your 239 out to 256, you took the compression ratio to 7.7. With '55 2 barrel heads, you would have 7.1. With '55 4 barrel heads, 8.1. With '56 ECZ-C heads, 7.36. With '60-'62 passenger heads, 7.7, but you are back to the '54 size valves. Remember, these are all BALLPARK numbers, not actual measurements. I don't remember, is your intake a '54 or a later one? Is this the leak you are trying to fix? Also, there may be some water passage issues when putting later heads on a '54. Ford used to make a special head gasket for this swap. I assume that making sure there are holes in the gasket where needed would be all that is needed. I once put late steel shim gaskets on a 239 and water leaked into the cylinders. Composition gaskets solved that problem. If you have the '54 intake, a change to a later one would make good sense if you change heads. Also, as a rule of thumb, Y heads can be milled .060 with relative safety. The intake surface of the head needs to be milled about 1.4 times the amount taken off the head surface. Hope this helps.
John in Selma, IN
John - "The Hoosier Hurricane"
