Hi CJ_Chevy,
Those pieces that fit up against the wood are separate pieces that bolt onto the bedsides (bolts are hidden underneath and the joint is filled with seam sealer). So even if you were to get new bedsides, they wouldn't include the pieces you need.
I was able to save/repair a few of those panels on dad's 77, but for the ones that were too far gone, we had a local sheetmetal shop make some replacements using the old ones as templates. I believe they used a bead roller to form the rounded edge where they meet up with the wood.
Before:
After:
The piece along the bedside is one of the originals and the ones that wrap around the wheelwells are fabricated replacements:
The piece along the bottom of the front bed panel is a fabricated replacement too:
