Now why would someone bury a GLC?
A few years ago there was an article in the daily scandal rag about a Ford Skyliner that was buried on someone's property down here, it was sort of an urban legend in that area. Sure enough, there was one buried there! The reasons for the burial were pretty hazy.
Anyway, the 1st gen RX7 axle is plenty tough for a Locost application, it's 57" wide wheel flange to wheel flange. The RX2/3/4/RWD GLC share a lot of parts with the 1st gen RX7 and it's narrower, the RX2 is 53" and the RX3/4/GLC is supposed to be 54". I haven't verified that one, though. The non-GSL-SE cars were all 4x110 mm bolt circle, GSL-SE's were 4x114.3. I had my RX2 axles redrilled for 4x100 and studs, much easier to find wheels!
The stock Miata rear axle setup is nice and strong with good brakes and a 4x100 mm bolt circle, its drawback is that with the shroud it takes up a lot of room in a Locost. If you don't mind fabbing your own control arms, shock mounts etc it would be an excellent choice. The R&P on the later Miatas is the same as a 1st gen RX7, too.
The 2nd gen RX7 (FC) setup has weird roll steer characteristics and the knuckles would be pretty tough to adapt to a Locost. They were 4x114.3 or 5x114.3. 3rd gen RX7s (FDs) were 5x114.3 too but like a Miata shroud and suspension linkage takes up a lot of room.
Another fairly plentiful axle would be the 7.5" GM in Vegas and Monzas (before anyone starts hollering, I know not all of them had this exact axle!) roughly 56" wheel to wheel, then there's the Fox body Fairmont/Mustang etc 7.5 and 8.8, the Pinto/Bobcat rear axle ~57" wheel to wheel, the Toyota AE86 and Celica, Datsun 510 wagon,