sounds like the braces are shot and shimming them is only a temporary solution. having looked a many parts trucks i have seen the practice of shimming done many times, usually with a piece of wood. if your driving the truck and not wanting to do the major teardown a piece of 1" sq. tubing welded in the cab mount area underneath the floor will stiffen the area up quite a bit. if your a purist, no offense with my bandaid solution.
