Wasn't really in the mood for much today (so I thought), so I started stripping...
Two hours of peeling paint with a razor-blade...
And then I got to the section just behind the locker on the nearside and noticed a crusty-looking bit, so I tapped it with the hammer and it scrunched, not unexpectedly. So I thought I'll just cut that bit out and fix it; cue cutting off the heads of 9 bolts (In retrospect, I only needed to do 5, because it turns out the part I thought I could slide out extends past the point I thought it did, and thus does not slide out.)
And when I peeled it back, and realised just how much overlap there was over the locker, and how it was never painted before assembly, I had second thoughts. New plan, cut the panel off level with the back corner of the locker and eliminate the rust-trap.
Like so!
Begone!
The locker portion still carries some of its DBG service paint, but
the other piece of panel? It's white primer and yellow over bare
steel; so it's purely from the commercial life. 
The body panel welded lovely, the locker utterly hated it. I don't
know why, but even after thoroughly beating it with the wire wheel it
still welded like utter curse word. As I said to friends at the time
"not bad for a vertical stick weld on manky old sheet metal. Pity it's
MIG."
Still, nothing a grinder and paint can't make disappear. I still have a little trimming and tidying up to do in that top left corner, but at least it's not packed full of rot now.
The panel isn't perfect, I managed to put an annoying wave in it while prying, and welding it certainly didn't help; some crude hammer & dolly work was done that hasn't got it perfect, but it's perfectly in keeping with the rest of the truck. The surplus bolt-holes will be plugged with bolts; they won't do anything but plug the hole, but at least it'll look sorta like it did before.
Oh, and I also painted the underside of the previously mentioned passenger floor panel. Debated painting the top-side, but it was already half six and getting distinctly chilly, so I passed on that.