The basic gears changed in 1951 from the earlier gearsets. If one was to swap the complete gears set from a 1951 Ford transmission into the 49-50 case, it's probably doable. Individual gears will NOT interchange from 1951 to 49-50.
As for the overdrive gears, most all of that is interchangeable.
The 1951 Ford rear housing has a different mounting flange for the rear support.
The 1951 shifter housing is also slightly different from the 49-50 Ford versions.