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.
