test a transmission out of the car?

Dont get me wrong I dont plan on spending ANY money on it.My scamp has a 904 already under it, and because I got it with no driveshaft it leaked all of its fluid on the ground when I went to load/unload it.Judging the by the color of the fluid its original transmission is probally "OK".Space is tight so I was trying to figure out if the spare is worth keeping. I'm just gonna get the scamp up and running on an old truck 318 and the automatic thats in there.Eventual plan is a 5.2 magnum engine swap complete with fuel injection and a 5 speed manual transmission, AND 3.91 or 4.11 rear gears.

I saw a video on Youtube where they hooked a drill to the input shaft and spun it while shifting the transmission through the gears and watched the tailshaft for movement.Is that possible to do or would it damage the transmission?