The L series gearbox's can't be used on an A series. The L series input spigot is of the larger diameter that the O.D. of the A series spigot bush. I used an L series 63A to replenish my fully fisted 63A A series box. I had to get a machinest (Dad ) to turn down the spigot bush journal to the same diameter as the a series one. As well as cut it down by about 15mm. (Approx)
The really tricky part is that there's nothing to hold it by. The only bit that could be held in a lathe chuck is the taper upon which the synchro blocker ring runs on. He had to machine a female taper to wedge the input shaft into to hold it. It was pretty tricky. I would have just used my old one, but it was a bit ratty. I stil have to old one. It's pretty handy for getting the clutch plate lined up perfectly.