Gene,
Your truck should have very thick rubber cab mounts that are about 1-1/2" thick. There are 4 bushings needed, two on each side, they sandwich the frame cab mount allowing both up and down motion as the frame twists under heavy load.
I got a hunk of scrap rubber from a local rubber supply company that they sell for the bottom edge of snow plow blades. Turns out it was the exact thickness needed and I used a hole saw to cut the round bushings out.
Mike B
