We tried to do the same with my O/D gearbox, but not very sucessful. We tried a different approach, by using an electric motor to generate the torque, driven via a rubber belt. Incidently, this is the same way we used to test A/C compressors before bolting them on.

While the gears shifted ok and we did count the revs on the input and output shaft, and everything looked good on the bench, the acid test is still to install it on the car and take it for a drive. Even when installed on the car on jackstands, the tranny/OD worked perfectly. But when the car was finally driven on the road, we found a few angry bearings that roared in 2nd and 3rd.
We had to pull the tranny apart again to change the bearing.

Sure, it did give me some confidence that the tranny was usable, but there are a lot of things that could go wrong with the tranny that the test could not uncover.
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...and the bricks keep on rolling