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The arms on which the cab is bolted have rubber bushings, but no adjustment that I know of. If the problem isn't a rotten rubber bushing, then it might be that the arm is cracked and bending down a bit. That happened to my 53. I put it on a new frame. I regret doing that, now. When I tossed the old frame, I also tossed out the VIN number.
Click on the link , enter cab mounts in the search bar and go to page 2, you can buy cast iron arms for $18 ea. far better than the original on the f100's I believe the bigger trucks came with the iron ones