I have a Tremec 5-spd in my 70 Road Runner. Same trans as in a Cobra Mustang. Went to a Ford dealer to get a tailshaft seal. "What car is it going in?" I told him it didn't matter, it is the same transmission as a certain year Cobra. "But what car do you have?" We went back and forth a few times, then I finally said "I have a 1970 Plymouth Road Runner". He said rolled his eyes and said "The Chrysler dealer is next door". I told him again that yeah, it is a Plymouth, but it has the same transmission as the Mustang. He said it wouldn't matter, they don't sell parts for Plymouths.
I found another dealer.