Here is what I have found, as I have researched this myself. A '57 to '59 Ford rear axle housing is the same width as the early Mustang (57 1/4"). The spring perches have the same spacing too (43", center to center). Those years actually have an 8 3/4" center section (ring and pinion). A 9" center section will bolt right in though. The axles are 28 spline, and will work with either the 8 3/4" or 9" center sections. Be careful, in '63 the 31 spline axles were introduced. Be sure to match splines if you plan to swap center sections. Also pay attention to fill plugs. The early housings did not have fill plugs. The fill plug was located in the center section. If you use an early housing with a later center section, you may not have any way to put in the the gear lube without pulling an axle and squirting 1 3/4 quarts of that foul smelling crap into the housing. If you don't want to mix and match housings and center sections, stick with the original 8 3/4" setup. They are pretty stout. There were 7 different ratios avaialble. 3.10, 3.56, 3.70, 3.89, 2.91 ('58), 2.69 ('58) & 4.11's. They had the axle codes 1, 2, 3, 4, 5, 6 & the letter "O" respectively. The ratio is also stamped on the tag bolted to the center section. Don't forget to grab the shock absorber mounting plates. The axle tubes are larger than the 8" in the Mustang. You can swap over the backing plates and brake hardware from the Mustang when you put in the new bearings and seals. I almost forgot, '58 & '59 Edsels are the same also. I hope this helps.