[Christ]

The average new gearbox is fairly small. As such, you could install one inline of the driveshaft without using the tailshaft housing or bellhousing/clutch.

You can build planetary gear reducers cheap enough and have "custom" driveshafts welded to be able to DIY in/out (once the setup is complete) and have your normal gearing within an hour when you need it, just by swapping driveshafts and installing a planetary gearset inline with it.

Remember that the further you get from 1:1, the more losses you incur, and the more moving components you add, the more losses you incur.

In other words, making a 2:1 planetary set would not yield a 100% increase in torque to the road, because some small percent of the increase would be lost in the friction and fluid drag inside the new gear case.