I bench tested my M41 before I fitted it. My donor car had been parked up for a decade or so and there was some doubt about the OD. I bolted it to a plate and clamped the plate in a vice. Then I made up a special socket to drive the input spline. My first idea was to drive it with an electric drill. My 13mm hammerdrill wasn't up to the task. It takes a lot of torque to pump up the hydraulics of the OD. In the end I used a carpenters brace to crank it over (like a crank handle). It doesn't need 1500 RPM to engage the OD, I did it with probably 120 RPM. I also borrowed a pressure gauge from work and connected that to the hydraulic pressure test port for interest. I hooked up the solenoid and cranked it over by hand to test it. I didn't really need the pressure gauge but it was encouraging to see the pressure get up to 450 PSI with it engaged, then drop to 20 PSI when disengaged. You don't really need the pressure gauge. I also clamped a piece of wire to the outlet spline and set it up so it struck a piece of sheetmetal on each turn. With that setup I could count crank turns while a helper could count output turns. Disengaged 1:1, engaged 3:4 - OD was confirmed to work OK. Then I ordered a shorter front shaft and proceeded with the conversion. It has been the best upgrade. Makes having a stereo worthwhile.

A bit of detail on the special socket - I bored out a piece of pipe to be a snug fit over the spline then drilled two holes in the wall of the pipe about 50mm apart. I cut the head off a 4" nail and bent it into the shape of a staple so that the legs of the staple could fit through the holes in the pipe from the inside. Get things right and the nail will engage in a spline slot and be sufficient to drive it. Then I welded a piece of round bar into the pipe and turned it down the 13mm so it would fit the carpenters brace. If this doesn't make sense let me know and I'll post a photo.