any C series gearbox off an A-series (4A, 7A etc.) motor should bolt up, only other thing to worry about is that the starter motor mounts on the correct side (later model c-series boxes have starter ears on both sides)
cheers gus so now i will attemt to find a c50 box hmm if only i wanst a poor student off to the wreckers i go!........ Unless anyone has one they are willing to part with for some spear moneyz