The original spacer pads will have probably allowed the cab to settle onto the frame. That's pretty common. The replacement (repop) pads are the right thickness, but they're not as stiff as the original stuff, so after a year or so you'll be back down on the frame (they settle faster). If you add the pvc spacers at a point you'll reach a situation of...what is it called...maybe homeostasis? Sounds right. What I mean is the point when it has settled as much as it will and both frame and cab are kept apart. I think the original pads were just made of better stuff. Hopefully this makes sense.
You can also use that pvc shower liner to make fender to cab pads, fender to inner wall pads, running board to rear fender pads...all kind of things anti-squeak and anti-rattle. It is tough and resilient and people seldom think to use it for auto applications, although Jerry and I have discussed it before and also Jon (carbking) and I discussed using it for carburetor gaskets as it is fuel resistant, and might allow for the take-up of some body warping. You'd just need a steel rule die to cut the stuff. I haven't even been able to cut it using a cnc laser machine.
