I forgot to mention the input gear train set-up.
You need to get a 20 tooth gear for the stud gear. Talk to Lurch
about this. You need a large idler gear, typically 80 tooth. you
need a 40 tooth gear for the stud gear end to get the coarsest
threads listed on the gearbox nameplate and you need a 56 tooth gear
for the gearbox input. The 40 tooth normally is used as a spacer
sitting on the end of the gearbox input when you aren't using it.
Most of these gears should be in your change gear stack already with
the exception of the 20.