I place 3/4" pieces of wood on the frame near where the rear cab mounts are for the rear of the cab to lay on. 3/4" is close to what your gap there should be.
I then lower the cab slowly while fishing the front cab bolts through the hole in the bottom of the cab and the frame.
Once I have the front mount pads and shims in place, and my cab support-to-frame gap is correct, I tighten the bolts.
I then install the rear shackle mounts and make sure the cab-to-frame gaps are correct.
I do it in this order to stabilize the cab at the front mounts, because the rear shackles are wonky it the front of the cab is not in the right position and secure.

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