I've got the GFI 5SR (Passon 5spd) transmission in my 64 Barracuda. The biggest problem is the lo-oong wait. It is a noisy transmission that sounds like an old Muncie rock crusher in the first three gears, (quiet in 4th and fifth though). I had to modify my tunnel a bit to get a right angle speedometer gear adapter on it so the speedometer wouldn't bounce because of the tight early A-body tunnel. Aside from those minor issues I love it. I got mine from John Huff.
Contact
John Huff
(307) 262-0125
He seems to be the guy that still cares about whether that 5SR transmission continues to be produced and has current information.