What I used on my '53 and am using on my '54 is a rear end from a '70 Ford F100. It's 61 3/4" from brake drum mounting surface to the other side. I have a 3.5 gear ratio with positraction or whatever Ford calls it. Anyway, a good place to have your differential gone through is Randy's Ring & Pinion in Everett, Washington. The do work internationally and have one humoungous facility. So you know what happens beyond this is I'm using 17" wheels with a 4 1/2 inch backspace from Billet Specialties. It leaves me with about 3/4" clearance between the tire and bed sides.

Dan
"I think I can, I think I can"
My Current Truck: http://ILovePortTownsend.com/54Toy.htm
My Last Truck: http://ILovePortTownsend.com/Truck_files/53.htm
My 59 Vette: http://ILovePortTownsend.com/Truck_files/59.htm
My High School Vehicles: http://IlovePortTownsend.com/Truck_files/late50s.htm