To the best of my knowledge, 1957 to 59 half ton Chevy frame, axles, linkage, shock mounts, steering box etc are the same. Both have short boxes although the 57 is a stepside and the 59 is fleetside. Everything mechanical underneath should interchange. Does that sound correct?
Yes and no...the mechanical stuff is the same, but the frames are different between the 55-57 and 58/59. The 58/59 frames are longer at the front bumper.
You will likely find that brake line from master cyl to rear axle assembly differ from 57 to 59. On 57 it crosses over to right side frame & back to rear axle. On 59 it goes back on left side to rear axle. FYI
delky dave in wyo
58 and 59's also had a stepside in addition to a fleetside. Rear backing plates are slightly different and use the wider rear brake shoes to match the fronts.