I don't remember having one of these apart lately. I would guess that in true GM fashion, the bedside is a bolt on, except for one weld at the bottom of the latch pillar, where it attaches to the rear cross member. It would be visible with the tailgate open.
If you remove the bedside, you can probably reach inside and drill out the existing spot welds between the upper and lower panels. Or you could just split them using an air chisel from the outside. Having the panel off would let you attach the new piece by plug welding along the previous spot welded seam. Or... you could reach inside with your handy spot welder and replace the OEM welds.
Going to this much trouble would probably be quicker in the long run, and your repair would be closer to original.
Just a suggestion.