There was not a pad or sound deadener on the back wall of the cab behind the tank. The kit being sold is for the two tank saddles & the two tank straps.
If you are stripping the cab, you can heat the back side of the cab wall with a propane or MAP gas torch, being careful not to overheat the metal. By heating the backside, the mat & adhesive is softened and will come off, easily with a flat-blade scraper. By heating the backside of the panel, you avoid catching the material on fire.
This method works great for removing liberal amounts of factory under-coating. Again, I apply the heat to the backside of the panel, to avoid direct contact of the flame to the material being removed.
When restoring a '60-'66 Chevy/GMC truck, I apply the sound deadener up the back of the cab, to the bump-out, behind the tank, for increase insulation & sound deadening.