Balance is important. Someone on Camaros.net bought a roto2000 and assembled it out of balance. The car wouldn't turn. Instead it hung heavy like a hammoc. Once adjusted, it worked great. Go with the arm dimensions Scott used in his design exactly or design in some adjustments so you can tweak things up.

Pneumatic casters are a good idea, as anyone who has rolled around a steel wheeled engine stand over a grain of kitty litter has discovered. I don't think I would try to transport the car while on the rotisserie though. The CG is still pretty high. The bumper is elevated to around 24" at the lowest setting. To load this onto a rollback, you need a different design that lowers down to the ground. That won't happen with a telescoping tube design.