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.
Yep. Section 2, Sheet 2.00. Models 3100.
It shows brackets riveted to the frame extending outboard just to the rear of the middle running board brackets. I just went out and looked at my frame, and that's exactly where the holes for the shackles are on my frame. Section 2 Sheet 1.00 (Models 3100) also references those brackets with a pointer that says "Body Bracket Instruction, Sheet 8.01.
Those outboard brackets at the rear of the cab do show two holes (each) that look like they might match where the shackles mount on 49-53 cabs, so may not require modifications, just some sort of spacer and pads. I have not seen a 54-55.1 cab to compare though.
You are correct on the additional work needed to convert the 47-48 cab from center rear mount to corner shackle rear mounts. I don't think that's a huge issue though as the floor is flat in the required location for the corner shackles (unless the 47-48 cab has more there than would seem necessary.
Also agree on the problem mounting an early bed on a 54-55.1 frame.
