Get a 1X4 about 4 feet long and lay it across the frame right where the rear cab shackle mounts are located on the frame. Your gap will end up right around 3/4" from the top of the frame and the bottom of the cab support. (a 1X4 is 3/4" thick.
Place your new front cab mount rubber pads on the frame.
Now lower the cab until it sits on the frame and the 1X4. I always start by lining up the front cab bolts with the frame bolt holes using a long tapered punch so that I can easily drop the bolts in place.
I then bolt the rear shackles on.
Go back to the front mounts and check the gap between the bottom of the cab and the frame to see if it is within specs according to The Book. Shim as necessary and tighten the bolts.
If you are lucky, you can now just barely be able to slide that 1X4 out and be done. If it doesn't slide out with a little bit of resistance, you will need to add or remove shims to acquire the 3/4" spacing between the floor and the cab. Those shims are located between the shackles and the bottom of the cab.

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