I ended up with a fairly OK used pedal box from 002spares, and rebuilt that one instead. Throttle linkage setup is certainly exciting.
Pulling out the carpet to get to the box, however, lead to a new discovery, and that meant of course I had to strip the rest of the interior to see how far it all went.
So that's rust in both rockers at the front of the doors with adjacent patches in the floorpans, rust along the tunnel to passenger floor pan join, and a large collection of holes in the rear floor and rear wheel arches where there used to be a roll cage bolted in (no sign of floor reinforcements, so one might assume not a very *good* cage).
Also, a surprising amount of sand on the driver's side; both having come in through the holes in the arch, and also packed in on the frame rail next to the pedal box.
With the rust also comes crush damage to the frame rails under both floor pans, and a wide assortment of other dents in the floor. It's time to take it off to a professional; this is more than I'm set up to handle.