maybe this is a dumb answer, but have you considered mocking it all up without gaskets? perhaps it has nothing to do with gaskets, its a pipe problem.?? Try it that way,, then......
I was reading some tips and tricks on another site about mounting exhausts on a tuber, the suggestion is to mount up everything LOOSELY, then go back and start snugging each part down, then snug each one again, and then tighten,, In sequence, front to back, or back to front, but start with everything loose, if you HARD mount one area, you can tweak the other sections and put stress and strain on it and thats when you see breakages, cracking and broken exhaust studs and mounts. It all stands to reason,, especially with the motor heating and cooling plus vibrating like crazy which these things all do. Play around with it a bit and check this idea out. Let us know how it goes, Looking fwd to seeing this completed and on your bike. :)