Unless the copper exhaust gaskets have been annealed, they are going to be too hard to properly seal.  I’ve a set I’ve been using on the various Y engines on the dyno and with well over 500+ dyno pulls on that particular set of gaskets and with a multitude of different headers and/or exhaust systems being used, the gaskets themselves are holding up fine.  The problem is that these gaskets have not been properly annealed and as a result, they are still on the hard side and still manage to leak exhaust at the heads.  Only the exhaust port edges have softened up at this point while the remaining portion of the gasket is still hard.

Here’s a link to a recent thread discussing the annealing of the copper exhaust gaskets.