Looking at the 1951 Chevy vehicle info kit, the 3800 rear shocks were RPO not standard equipment. I'd assume that GMC's were the same. The FAM shows upper brackets rivet to the inboard of the frame on 3800's with PN 3705695 (LH) and 3795696 (RH) with a lower mount welded to the axle tube. Those same upper mounts were used on 4000, 5000, and 6000 series Chevys. So maybe you could find a pair of upper mounts on a bigger truck.
