Hmm, you shouldn't have to shim the cab more than the factory specification, so here's some thoughts....if none of these apply, I'm stumped....
When you replaced the cab mounts, did you shim between the cab and rear shackle on each side to get the correct clearance between the frame and the floor panel of the cab ? The distance between the cab and the floor panel on your truck is .760" +.060 / -.0 .
On the front mounts , the distance is 1.830 " +.060 / -.0 .
You can go online to the chevy old car manual site and review the information on cab mounts, if that's helpful.http://chevy.oldcarmanualproject.com/shop/1954truck
I recently replaced the front mounts and refurbished the rear mounts on my 1954 3604. It was a bit of a challenge.