+1 for the Autotwirler, for all the reasons already mentioned. The quality and customer service are first-rate.
Unless you can buy the steel on the cheap / get it for free, and have the time to fab one yourself. I was going to build one, but my wife saw me looking the autotwirler over at a Goodguys show (to look at how it was designed, etc) and she went back to the guy and ordered one for me. (Yes, she is definately a keeper!)
Regarding firewall work: The body dolly that they make is also extremely handy for situations such as this. It comes with adjustable height stanchions with clamps which attach at the pinchwelds on the bottoms of the rockers. Highly recommended. -Rob