I'm running a rear end from a '74 or so 4X4 in my '57 GMC half ton. The spring pads were about 3/4 of an inch farther apart than on the '57 axle,but there's enough flex in the spring bushings for it to fit. the total width was close enough to clear everything fine. I had to do the u-bolts differently,I used the newer ones,and the newer speing plates.I had new shock mounts welded to the axle housing and used the rear shocks from the donor truck. I used my original parking brake cables,so that still works too.
My original axle was a 4.10,the replacement is a 3.08-almost TOO tall for the 270.
My point is that the differences from '57 to '74 axle assemblies are few enough I consider it a bolt in swap.
Speed
