Sounds like they have it set up to just lay the cab right on the frame, and that the front crossmember is set at the correct position for the radiator support which will allow the front clip to fit.
Is there any reference as to what to place between the bottom of the cab and the frame? I am guessing that it is not supposed to be metal to metal.

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