I think I found the bottom rubber. KEEP ALL THE METAL PIECES OF ANY KIND. SMALL AND LARGE. DON'T THROW AWAY ANYTHING!!!!!!!!
No.'s 9 and 10 are definately worth a
TRY.
The bottom rubber of the rear mount for manual trans 1960 GMC is listed in Rockauto. They are out of stock. The number comes back to 54-89 Chevy. MM 20-A. The dimensions are Approx. 2.5" long, 1" thick. Don't know width. The top may be a DIY "make it fit". Use all your original metal backers for top and bot as you can. Bolts too.
The MM is Metro Molded Parts. Was in contact with a nice lady. They only have one. They can mold more but at $30 each shipped, forget it. Go with LMC, cheap enough to try unless you already did. Actual GMC mounts for your truck are probably not available.
THIS is back when I was a young kid. I have since sold that 1960 4x4.