Frame is supported by suspension and springs. Height off ground is only part of the data needed. Are all tires the same and fully inflated? Wheel bearings O.K.? Springs intact? Shocks good? Brackets and mount points symmetrical?
1/4" here, 1/8" there...imperfections can "stack up" against you, and with the work done you described, there's a few extra places for things to get "off". I wonder what the new truck tolerance was for rear frame rail height in the early 50's...+/- 1/2"?

If you can't fix it with a hammer and screwdriver, you need a bigger hammer.
1965 Chevy C10