This is a store bought one from HF that isn't too expensive. I think that I bought mine for $75 with a 25% coupon. Works great on the SM420 on my '57.

http://www.harborfreight.com/450-lb-low-lift-transmission-jack-61232.html

It is make it easy to slide the tranny out of the way and then to spear the shaft into the pilot bushing when reassembling. The adjuster on the scissor lift lets you precisely adjust the shaft to the pilot bushing. Makes removing and installing an easy one man job.

The only problem with it is that the tranny is so tall that you have to jack one side of the truck up very high to roll the transmission out from under the truck if you want to work on it.

Matt