What condition are the front cab supports on both sides? Same for the floor boards!
If they are sound, I would suggest that you remove the rear shackles and place 3/4" boards between the rear cab and the frame near where the shackles belong. Then add shims on the front mount which is light in the loafers until the the two mounts have the same amount of cab weight on them. Tighten the front cab mount bolts.
Now install the rear shackles and remove the boards. The cab should now sit level. If not, then there is something wrong with one or the other of them or their mounting holes in the frame.
