I did some digging around for info on that system and it really doesn't look that useful for the amount of work it would be to refit a car with it. All the cars that have this system have a taller final drive gear as well so any gains it gives by itself are going to be pretty marginal.
A much better system for bigger gains is to use one or both of the rear wheels of a fwd for direct electric drive. With that the starter could also be removed and just bump start the motor with the clutch when needed.
But for some cars like LostCause's tbird there could be an even neater setup, since the differential is solid mounted on that car and the u-joint flange is bolted on it would be easy to put a sprocket between the diff and flange and run the chain up into the trunk for an electric motor. Toss the batteries in the trunk and there is an instant hybrid
If the car is a 5spd then the gas motor can be turned off and roll around in N without wear.
Makes me wish I still had my old 93 tbird to play with...