Two days ago at the Mopar Nationals I came across this guy:
http://www.theramman.com/products/power_brake_kit-midland_ross_66_70_b_body_complete/index.html
Note that he does a lot of things with brakes and is quite knowledgeable. I told him you were restoring a 1965 Dart and needed to know if there was a gasket between the booster and the MC. He said "Absolutely Not! Chrysler NEVER used a gasket there because it would affect the brake rod adjustment." He said that over the years, people started to make gaskets for things like this and over time, the issue has become muddy. You might want to send him an e-mail and ask about your other question regarding the nuts.
I also asked him about the correct master cylinder to be used on my friend's 69 GTX w/ discs and off the top of his head, he rattled off the casting number that should be on the bottom of the MC. He had the correct one sitting right there on the table and sure enough, it was xxxxxxx. If you have any questions about your brakes, he's the man. He also sells individual parts.