Some of it depends on where the rings are. You can only get the piston up in the bore as far as the rings lest you. But .060-.080 usually isn't an issue.
Measure the recess in the heads before you start. About the deepest I've seen them is .110 but most by now if they've been surfaced will be in the .080-.090 range.
Get the heads flat and equal first, then decide how far you need to get out to the hole. I wouldn't be afraid to call Cometic and get some gaskets that are .027 thick and use them. It will take less milling of everything.
