Idling motor with little fluid in trany ?

Yes it circulates in park. Running it dry (or real low) will most likely trash the transmission. Things in there are spinning even if it isn't in gear. Bushings and seals don't like to run dry.

If you want to do it without making a mess or hurting the trans get a yoke to slide into the rear of the trans. and keep it in park while your running the engine and put at least 7 qts. of fluid in it. Then when you install it in the car jack the rearend way up before you take the yoke out to install the driveshaft and you'll have very little if any ATF spill.