all I can say is that the rear end I put in my 50 f1 is 61.25 from the wheel mount to wheel mount I can go and measure my 50 if you like. ED I get 12.5 in inside the fender to the bed of the truck at about 3 in above the tire. I believe it was from a 9" 73 Mustang.
On my 51, I had more problems with interferance against the box than the outer fenders. I would mearure the fenders for you, but the truck is in storage ar my daughters house for the winter. Next time I am there, I will try to remember to measure it.