With the 3100 at least on my '51, nothing bolts to the bed sides except the rear cross sill. The second cross sill bolts only to the wood and the angle strips, and floats above the frame. I could see where the second cross sill with the brackets on the end could bolt thru the bed sides. Maybe that was a 3600 feature only.

Kevin
1951 Chevy 3100 work truck
Follow this saga in Project Journal
Photos
1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.