If the rust perforations are the only major rust problems on that panel, why not lead/bodysolder the repairs instead of trying to locate a 1/4 panel to cut up to repair it. I had similar problem with holes on my 1/4's where there had been a vinyl roof fitted at some time. It's far easier/quicker to lead/bodysolder the area & it'll be a permanent repair without the worry of rust re-occuring. Here's some pics of my repairs, hth
more pics here
https://lateral-g.net/forums/showthread.php4?t=6340