Looking at the Factory Assembly Manual, the inner panel spot welds to the turned down front of the belt rail after that, the upper outer and lower panel are welded, so like Carl said, unless you removed the inner upper body panel, the joint for the outer panel would be basically inaccessible, although it could be plug welded from the bottom. It would definitely be a PITA. Maybe a butt weld joint would be the way to go. I'm guessing the need to replace may have been dictated by the rear of the cab being beat to a nub by people throwing stuff in the bed and likely rust on the bottom as well.
[on edit] meant inaccessible above.

Last edited by klhansen; 10/20/2017 6:30 AM.

Kevin
1951 Chevy 3100 work truck
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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.