Send the input shaft to me and I'll use a proper grinding machine to reduce the input shaft diamater. Lathes are inprecice for this sort of thing. At the very least you need a monoset to do the job. I can't remember who has done this before on here but they have used the stanza input shaft. There is no reason why you can't just make the brass spigot larger to accomodate the input shaft of the l series but you will need to shorten the spigot length on the input shaft.

If you need, I'm happy to do some grinding... I think I know someone who works at a place that may build gearboxs.......