Milling would raise the compression.
Thicker head gasket would lower it back down, maybe to where it was before milling and possibly thinner old head gasket.
You need to know what head gasket was on it before and either use the same type or mill to compensate.
After that...measure for push rods if yours are too long
I know that milling will raise the compression but if u don't mill the valves then u will not seat the valves because it is non adjustable rockers u cant adjust rockers get there oil from the block casting not through the push rods
