First, you need to determine what's not level. Assuming your floor is flat, start measuring to the floor and comparing from side to side. Once you have the frame blocked and level, then check the cab mounts. They probably need replaced, but you can shim up the cab off the frame. If it's the cab itself, jack and block up the cab until it's square with the floor, too.
As long as your shop floor is flat and the frame is square with the floor, you can square the cab with the floor be measuring. Even if the shop floor isn't level, you can still get everything squared this way.
