[HO455]

Where are they being used? The large cab mount bushing? I am pretty sure that they factory didn't use different thickness washers or shims. Once the cab was basically dropped on the frame everything else got adjusted to match the cab. GM manufacturing processes were meant to be quick so having a bunch of shims to try and align the cab perfectly didn't make financial sense.
Now in the 50 or so years sense some could have been added after an accident or for other reasons
My Burban had no shims installed when I replaced the body mounts on it. (See my nice collection of rust!) As long as the body on a Suburban is GM saw there was no need for shims. So I can't see how a truck cab would need them from the factory. Unless the body mount was bent during assembly and needed to be cobbled up to get it out of the door.
Now I could be wrong if someone knows for sure please correct my theory.