Valve Cover Gasket Sealing Issue

On my 340, I replaced the heads with 1.88 valves with 1971 heads with 2.02 valves. I was going to use the Felpro cork gaskets from the gasket kit, but I noticed that both heads seem high in the center, so when I push on one end of the valve cover the other end has a gap.

Has anybody else encountered this situation? If so, did thicker gaskets solve the problem and which ones work best?