The driveshaft is a custom made one with what I was told was a yoke off a 120y on the gearbox end.
Do these two parts not go together? If so then that would totally explain the problem.
I have a vague recollection that my original gearbox didn't have the copper coloured bit on the back of the gearbox (dust cover? or seal?) which would have let the yoke go in another ~20mm.
What is it meant to look like on an original 1200 or 120y?
I guess this is the problem with putting together a bunch of parts that weren't designed to go together.