You can measure check the cab height prior to putting on the front clip. The front cab mounts when assembled correctly are a known reference point. In the Salesman Data Book it gives a top of frame to top of floor as 1-7/8" you can check that at the rear. It should get you very close. The most shims I have ever seen on cab shackles were two which is about an 1/8" thick.

Shackles with shims.
http://www.flickr.com/photos/advance-design_parts_co/8129510232/in/set-72157631868452754