55.2-59 1957 Stepside Bedside help needed

[Oldtruckwelder]

I’ve got a pair of bedsides that definitely needs the forums help.
Years back i added new bedside strips with plug welds and new strip of metal to fix rot. Covered them in rattle can primer to protect and stored them.
Recently got them out to prep for epoxy primer and bodywork.
Checking closer with straight edge found both panels had large high spot with tight oil canning… I assume from weight of fender hanging lots of years and possibly stretching metal. For a test I bolted one of the fenders on and it captures the oil canning but doesn’t fix problems..
Marked some pictures to try explain it better.. not sure where to begin.
Need you advice please
Thanks
JT from SC