Firewall & master cylinder don't match?

I'm making the same brake change on my GTS. Just looked at it and see the same thing you are talking about. I have a paint gasket kit laying around here. I just looked and I have what looks to be the thin paper gasket for the drum application with the oval cut in it. It covers outside the oval just a little. I am going to put that thin one on first and add the foam one for the disc set up. Looks like it will seal fine. If you don't have a gasket set, get one.