The floor section above the no. 1 body crossmember (passenger's side) -
After drawing up a cardboard pattern, it wasn't that hard to reproduce in sheet metal. It's mainly parallel folds, with a raised portion in the middle. When I had the section largely bent up to shape, the outermost folds were easily enough done with the piece in place.
So I red-oxide primed the inside of the no. 1 body crossmember as far as I could get the radiator brush in, and also primed the underside of the floor repair. Then holes for the plug welds -

And I adjusted the gaps. But a few of them were wider than I'd have liked. Having a part of the floor cut away was a good thing, because I could G-clamp the repair section in place while I did the first few plug-welds. A screwdriver jammed into the gaps at the right places sat the repair down hard onto the flanges of the no. 1 body crossmember for the rest of the plug welds.
Gentle massaging with a hammer to line the edges up, and a couple of feet of weld. Some work with the grinder, and then a quick coat of black primer:

And then the sun went down on me.
Thinks: They tell me that American Autos in Prospect has shut, and that all Rare Spares inquiries are handled through Brisbane. I was thinking of picking up a driver's floor pan. But now that I've got the cardboard pattern, and now that I've made one repair section, it won't be that hard to turn the pattern over and make another for the driver's side. It's the most complicated shape I need to make for the driver's side too.
Rob