resurfacing gt 40 head question help

You pretty much need to do both heads. Not doing so will misalign your intake manifold to your new head and could cause all kinds of issues. If it's only a .010-.015, you might just get away with it, but either way most shops will charge you the same price to do both and it's extra insurance to ensure the deck surface is true on both head and you don't have this problem again.

Yes, it will change your compression some, but we're talking less than half a point here. Not enough that you'll notice a huge difference. Try to keep it .030 or less if you can. Any more than that and you may have to mill the intake down as well for it to properly seat the gaskets. As it is you may have to elongate the intake bolt holes, but that can be done with a rat tail file and a little patience. But you won't know until afterwards if it's necessary.