The Allison really doesn't feel too heavy. Everytime I've R&R'd one, we just used a regular jack, and I held on to it to stabilize it while someone controlled the jack, that way if it slipped off I could hold it there. Never had a problem lining anything up either. I could see being on a lift adding difficulty though since it would be much higher up.
Screw laying on your back and putting a tranny in. With the right equipment its alot easier on the lift.