First, a bit of terminology: the combination reduction gearbox and differential is a 'transaxle'; the combination of electric motor and transaxle (usually with controller/inverter as well) can be called a 'drive unit' (at least it is for Teslas, and other brands have the same arrangement).
The Leaf is FF and I would prefer a FR car as it was made RWD from the factory. What is the ideal setup for transferring power from a Leaf motor to the rear wheels if a 2-speed Powerglide is not optimal?
There are at least a couple of people building projects which place the entire Leaf drive unit in the rear, in place of the original final drive (differential) to use the Leaf motor (and transaxle) in a car which was front engine and rear wheel drive. That has the advantages of being able to use the drive unit unmodified, avoiding the need to build parts to mate the motor to some other transmission, and leaving the engine compartment free. Unfortunately, since the car wasn't designed to have anything as large as the Leaf drive unit there, this is a major project involving structure and bodywork.
Example project:
300ZX Electric Conversion
At the opposite extreme, the traditional conversion approach is to just put the (Leaf) motor where the engine was, connected to the car's original transmission (which then still connects the original final drive (differential). The only reason to change the original transmission to a modified Powerglide would be to avoid shifting, but most people just shift the original manual transmission as necessary.
A compromise is to put the motor and a small single-ratio reduction gearbox in the original transmission tunnel, driving the final drive unit (diff) the same way the original engine and transmission did. If using a low-speed motor (not the Leaf) or building a car intended for high speeds (not regular street use) the reduction gearbox can even be omitted, so it's just a motor connected by the driveshaft to the final drive (diff).
One builder in this forum even managed to fit a motor in directly ahead of the final drive (essentially in the driveshaft tunnel under the rear seats), connected to the final drive by just a short coupling and with the motor housing mounted to the final drive housing by a custom bracket (which acts like a short torque tube):
Latvian mazda RX-8 project
I'm quite new to the world of EVs, so my understanding is lacking. So, how exactly would I connect the Leaf motor to the rear diff? Torque tube?
In any solution which involves keeping the original final drive (differential), you connect to it the same way as the original car: you use the original propeller shaft (driveshaft), modified if necessary to fit your chosen gearbox (or directly to the motor).
A torque tube is a non-rotating box-shaped or tube-shaped structure which connects two parts (such as an engine in the front and a transaxle in the rear, or a transmission in the front and a final drive unit in the rear) to handle the torque in reaction to shaft torque. In a car which doesn't use a torque tube, those parts (engine, etc) must be mounted so that the body can handle the torque and keep those parts from twisting... and that's the normal setup, which is why final drive (differential) housings in typical cars have two widely-spaced rear mounts. As far as I know, no Nissan has ever used a torque tube. Of course anything with the engine and transaxle at the same end of the car (FF or mid/rear engined) has no torque tube because there is no distance to fill between the parts.
