BTW, there are still some small mods needed to fit the TR6 box in the 3A. Longer studs in the back of the engine, move the holes in the rear crossmember, that sort of thing. Should be no problem, but it's not quite a drop-in operation.
First time I did it, I didn't realize that the 4-synchro box is a bit longer than the original 3-synchro, and just forced it in. For some reason, my motor mounts didn't last very long ...