Agree with Paul on the threads, also, did you spray anything on the gasket before installing it? On copper head gaskets, I always spray them with cheap aluminum color spray paint (the kind that is dull and rubs off on your hand when touching it!) It allows the surfaces to slide a hair when torquing the head down and seems to help the gasket seal better. It may be useless but I was taught to do that on copper head gaskets and never had a problem on my old flatheads running them. I also did the same thing on the steel shim head gaskets on the 440 in my Barracuda.
