When I switched my 69 from FMX to a C4 the drive shaft had to be shortend Im pretty sure. I had a spare that they cut and welded back together. I remember looking at the gap between the tail of the tranny and the yolk of the stock shaft and that there wasnt enough room.
Guess Im not much help, Id wait till someone else chimes in, or ask a tranny shop how much space there should be then after you swap the trannys just put the shaft in and check the distance and then youll know, take it to a machine shop and have them adjust it. Done.
Don't know why you'd want to swap an FMX for a weaker transmission for, but they're close to the same length, but the FMX uses a big block output yoke, so you'll need a smaller 28 spline yoke for a C-4.