Are you using dowpins? I would check the block deck with a straight edge. I may be warped. Or maybe you have grit on the gasket surfaces or a bur holding it up. One of my friends had a problem with some bracket catching one the back side of the block holding the head up. Just look for all the dumb stuff. You may have a vac line mixed up with a coolant line. All kinds of crap can happen so just think of all the things you haven't already done.

A good gasket surface and a good gasket should not need coper spray.