216 heads are notorious for developing cracks. I'd have a good machine shop do a .010" cleanup cut on the gasket surface, and do a magnaflux crack inspection at the same time. Heads can tolerate a lot of milling with no ill effects, at least until the intake valve to piston clearance gets too close. Up to 30 thousandths or so shouldn't be a problem, but don't take off more than is needed to get the gasket surface flat. Up to .005" is probably acceptable without doing any milling on a head gasket change, but as always, "the flatter the better". Don't forget to check the block with that straight edge- - - - -it's pretty rare, but not impossible to have a warped top surface on the block.
Jerry
