The 47-54 FAM (Sect 1 Sheet 3.12 for all 3000 - 4000 - 6000 cabs) points at the shim between the floor and the cab mount shackle and say "USED TO MAINTAIN .76 +.06/-0 DIM BETWEEN FLOOR PANEL AND TOP OF FRAME"
For the front mounting point (Sect 1 Sheet 3.09) it calls out a dimension of "1.83 +.06/-0 TOP OF FRAME TO BOTTOM OF FLOOR PANEL 3000-4000-6000)"
That difference is interesting as the cab welding checking dimensions (Section 1, Sheet 1.07) indicates top of floor at 11.88 and top of frame at 10 above the reference line. That indicates that the floor is parallel with the top of the frame, but the rear shackle mounting dimension would have the rear lower than the front by over an inch.
Anyway, a rigid mount should maintain the same dimensions, assuming the frame configuration is the same as a stock frame.