I use a Staton Inc 18.75:1 reduction gear box. The engine is a RS 35. I'm a large person 250 lbs and live in the mountains. Therefore I need gears and use a shift kit. The reduction range is 49.21:1 to 18.82:1. In the future I plan to upgrade the engine to a Honda GXH 50. Then the reduction range will be reset to 43.42:1 to 17.88:1. With the Honda 50 I'll have a little more horsepower and torque is why I'll adjust the reduction range.
I choose the Staton gear box because it uses gears instead of belts or chains. Making it a very strong system for carrying a lot of weight up steep hills. My bike also shifts 7 gears automatically.
With all that being said the BB transmission is hopefully strong enough to do the same. If BB has been like it has in the past you may need to upgrade the freewheel bearing. Still I do like the fact parts can be replaced in it.
With a 6.83:1 reduction, a jack shaft and a shift kit one could build a good shifter bike. As to what your reduction range should that'll depend on engine choice, overall weight and terrain. I too hope more responses come onto this thread.