On the '55-'59 chassis, the frames are much closer in construction than on the earlier trucks. They maintain a 34" dimension across the fram width. The primary differences are in the length and the thickness of the material. For the most part, 1/2 ton suspension peices can be swapped directly into a 3/4 ton chassis. The front suspension is virtually identical, only having a few more leaves and the axle forging and spindles are heavier. On the rear, there's not really much difference other than the axle, depending on how the truck was optioned. The base 3/4 ton uses the same springs as the heavy duty option on the 1/2 ton. Take a good look under a 3200 sometime, they're essentially a 3/4 ton with 1/2 ton axles and brakes. Sort of. The front hydraulics in a 3200 are the same as 3/4 ton.
I would also suspect that the GMC would have the Spicer rear as well. If it does, that makes things much simpler in the gear department, as there's lots of gear choices available for that diff.
