When I dragged my 51 3100 home last May the under bed spare tire carrier was complete intact with an old spare up in it. I took these pictures to remember how it goes back together. It also had a unusual nut that was hold the rack up against the tire. As seen in the photos. The bolt with the flat steel at the end dropped down through a hole on the very back crossmember of the frame and was held in place with the cotter pin. Both frame crossmembers for the tire rack can be unbolted from the frame. The very back one is channel up. Second hole from the end of the frame. The one towards the front is channel down. Forth hole from the end of the frame. Hope these help. If you need more pics let me know.
Jeff 1964 Chevrolet K10 Fleetside "A Kid's First Truck" Follow in the DITY 1951 Chevrolet 3100 Follow his build in the Project Journals 1969 Chevrolet Camaro Convertible | 2020 Chevrolet Silverado | 3500 Duramax | 2021 GMC Sierra Denali USAF Veteran 1983-1987 | PSP retired 1990- 2012