An explanation of the difference may be that just about everything in the FAM is centered around the '54 models. That drawing you referenced is dated June of '53, so it's possible the specs changed a bit from '51. If you look at the cab weld checking instruction Section 1, Sheet 63.02 in the FAM, it says dimension tolerance is +/- 0.02 inches or just under 1/32 inch. I'm sure there's similar tolerances (or even greater) for fender alignment.
Yep, use those as a starting point and adjust things to fit correctly.
