Hey Richard,
Which gearbox?
Without knowing that, it's nearly impossible to comment apart from a few things.
I've offered up my 5-speeder and it interferes with the horns for the 336 rubber mount - 5-speeders are longer. I made a new 'hoop' to accommodate that and the different mountings - a work in progress.
You'll undoubtedly need to fabricate a new gear change from the cross-tree back or convert to a cable shifter.
Might need to modify / change the clutch actuation.
Also might need to check if the gearbox output drives are the correct length.