D I have beat this up quite heavily with my machinist pal; if like him I had the ability to machine parts it could indeed be a budget set up. There are few ways to do this as mentioned. You could use the bike box and a Quaife reverser box. That would be about $2000 for parts. You could use the Datsun bell housing and input shaft to drive the bike box then use the Datsun center plate and tailshaft housing on the output side. That could be as little as $750 in parts.
The maching bill would, even at super discount rates, be an easy $2000 and more likely $4000. At the end of the day we'd have a box that could cost $6000, which brings us to the point where you could buy one outright.
It's a fantastic idea but sadly I don't machine, I wished like many bikes and cars we had an off the shelf option that didn't cost as much as my car.

Tom