I put a 88" IIa tub on my III, as the original tub on my III was heavily corroded, and the IIa tub was in much better shape. The big difference is that there are tabs at the front of the tub on a III that weren't there on the IIa; you can fabricate some there. The rear of the tub bolts to the chassis the same, the seatbox bolts to the tub, so it works. The covers for the lights at the back of the tub are a little different, you can use whichever set you want from whichever tub.
The doors, the wings, most everything works. The bulkhead's different, and thus the dash, the front door hinges are different, but other than the transmission synchros, they're basically the same vehicle.