I don't have one to measure, but I'm pretty sure the inside backing plate to backing plate measurement is 50 1/2 inches. If you're looking for a good axle to fit, check a 74-80 Mustang II (6cyl). The housing measurement from end to end is 52". I've used them in 2 A's. It is an 8" with the pumkin removed from the front. Fits in perfect although you have to have the axles drilled for your bolt pattern.