The gap for the rear cab mounts is measured between the top of the frame and the bottom of the rear cab support bracket, not the bottom of the cab.
Best way to set up the rear cap mount gap is to place a 1"x 4" on the frame near the cab shackle vicinity and lower the cab.

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission