The '55 to '59 trucks were optioned with the V8 , so their diffs must be strong enough for average use .
I have a '55 rear in my '40 and it works real well (3.9:1 gears)
Check what brakes you are using in the front , they need to match the rears in efficiency.

http://www.stovebolt.com/techtips/55rear.htm