Jon, thanks again for the information. I'd probably have come to you for a carb, or at least a kit, but I don't think you ship to the UK.
Comparing the carbs, I'm pretty sure they're both the same - or at least, the same variant. The 'spare' is marked 0-456 on the throttle body and stamped 456 on the mounting face, which the original isn't. The tops are a fairly lousy match to the bottoms on both carbs, but all the internals seem to match OK - the castings just don't match cleanly on the outside, but looking at photos this seems to be common.
Anyway, the spare carb seemed in better condition and wasn't corroded in the bottom, so I rebuilt that using the venturi that came with it and the jets / nozzle from the original truck carb.
Put it back on, the truck fired up and idled cleanly immediately. Picks up cleanly too. Haven't driven it yet, so we'll see how it goes.
