I had done some research on this issue before and here is what I found - in the factory assembly manual it shows on sheet 5 of section 11 that the rear reinforcement pad is GM # 3664766. Its glued to the rear reinforcement using an adhesive cement. The pad 3664766 as shown in the master parts book X-16400 dated September 1963 shows the pad to be a sponge rubber pad, 1 1/4" wide x 10" long and 7/32 thick, and it requires two of them. Funny thing is that this same part number is also used to describe anti-squeak pads used on passenger car hoods but shown in a different length. I suspect the 3664766 is the part number for the material and they cut it to length. Hope this helps. Resurrecting 62 + year old trucks is more fun that should be allowed!
