If you swap the axle, best would be to get a matched pair from the same vehicle. After '63, the 3.90/3.92 gears don't show up, 3.73 became to common gearset used. Also, since the rearend isn't stock to the truck, it may not be matched to the front to begin with. For best overall upgrade, look for a '71-'87 axle set, which will have open knuckles and disk brakes. The open knuckle front axle is stronger that the closed knuckle. '71-76 will be a Dana 44/12 bolt GM combo, later trucks used either a 44/10 bolt combo or a 10/10 combo, depending on year/option.
