(Not pictured: the obligatory beverage)
Go round in circles for a bit, and voilà!
I didn't quite get to place the fuel pump hole where I'd originally planned, due to conflicting with the fuel tank straps. It's sat further out than I would've liked, but I didn't want to end up running into the level sender and causing myself even more headache.
(Please note, in the background, the patent-pending explosive atmosphere detection kit: a burning wad of paper on the end of a welding rod.)
On the level sender note, I went and dug out the universal ("One size fits Fuck-all") GM tank sender that I bought quite a few years back now, and set that up with the same angle of dangle as the original.
May end up tweaking that float arm a little more, in the future, but for now it's good. Inside of the tank is also in lovely condition due to being galvanised; there's some sediment on the bottom -- plus all the swarf from drilling, now -- so I'll wash & hoover that out as best I can.
Then comes the weldinating!
I should've cut that bung out, I really should; it turned out to be soldered in, and that made a real mess while I was welding so close to it. Still, it's all welded in and solid. Once it's cleaned and painted, it'll be golden. Well, black, but you get what I mean.
Weld up the screw-holes for the original sender, so I can drill the flange for the new pattern.
Quick spritz of zinc primer, just to stop the lot of it going crusty
before I get around to cleaning and painting. Mostly just a wash off,
and a quick buff over with some sandpaper to key it.
Test fit! 
And, of course, a little jewelry... Gave the cap a bit of a
polish.
I'll need to make up a gasket for the sender, and a new seal for the filler cap, but I'm not expecting either of those to be a particularly difficult task. (Must also get some rubber to put behind the tank-straps, too.)