Right you are, Idk, reverse is...reversed.
Problem is, cars typically have the final drive pinion on the right side of the ring gear, and the R3 pinion is on the left. There are three possible solutions I know of:
1) Turn the auto axle over, left-to-right. You need to block off the vent, and emptying the lube will be a hassle, and on most axles it will raise the driveshaft.
2) Use a front axle (or front diff, for independent suspension versions) from a four wheel drive vehicle. They already go backwards.
The problem with both these solutions is it puts the drive shaft on the driver's side of center (for left hand drive cars, like in the US and Canada and such) which does encroach on cockpit space a bit. Leaving us with...
3) Design a new rear plate for the output shaft, and an adapter to connect the drive shaft directly to the transmission output shaft. The way it works on an R3 is there's a drive reverser built into the back of the engine, to accommodate the needs of the final drive in the wheel. In theory, this should be relatively easy (that is, relative to the difficulty of reversing the rotation twice).