Looking at your pics, I think that your rear cab spacing is close to correct. I suspect that your measurements were taken from the wrong point.
A 1" X 4" (which is actually 3/4" thick) should barely fit between the top of the frame and the bottom of that c-channel support bracket.
Did you happen to take your measurement from the inside of that c-channel instead? That would give you a much larger number.
Here is how I mount a cab:
Place a six inch long 1x4 on top of the frame near where the rear shackles mount. One on each side.
Lower the cab so that you can install the front cab mount bolts through the frame. Put the square rubber front mount pads in place and put the nuts on and tighten hand tight.
Install the rear cab shackles. If you cannot slip the 3/4" thick 1x4s out once the rear shackle bolts are tightened, remove the top shackle bolts and add a shim and slide the 1x4s out. This will get you real close to the correct specs.
Once the rear of the cab is spaced within specs, measure the front cab mount spacing and shim as needed to be within those specs.
