Bottom line is that NONE of the 5 lug rear end gears, spyders, axles will interchange with your '51 truck. While the design is the same in pictures and parts lay out, all the truck 6 lug parts are slightly larger that the 5 lug passenger stuff. Longbox55 suggestion of redrilling the axle flange to 5 lug may be the best way to go. Best thing I did for my old half ton was switch to open drive, modern brake come with it and lots of better gear ratios.
