I don't know the facts but I can't imagine the cab mounts would be different from 1/2 ton to 3/4 ton, I just can not see that at all. The FRAME mounts that hold the cab up, "MaY" be different, but the rubber mounts should be the same.
It is very common for the cab and bed to not match. If you need cab mounts, to it. Don't "expect" it to fix everything, just do it, because it needs them.
After you have installed the new mounts, than you can shim the bed or cab to make them fit perfect.