From what I can tell by looking at the FAM diagram for the '54 frame, the rear cab mounts are similar to the front mounts (rubber donuts and thru-bolts) instead of the shackle type rear mounts on the 47-53 body. A couple options come to mind for what you're considering.
1. Drill and reinforce the floor of the 47-53 cab for the later style rear mounts.
2. Remove the later style rear brackets from the frame and drill for the shackle rear mounts for the earlier cab.
I think #2 would be the easier route if you wind up with an earlier cab.
Are you sure that the FAM diagram was for a 3100?
All of the vendors are selling the same mount parts for '49 through 55.1 for rear cab shackles.
