Hi Art,
I have been following your posts here knowing what you have planned. I built my own a axis, or better said, I designed it and had it built. I do not do nearly as heavy work as you do but I want to give you my input as it might be helpful to you. I used a 300 oz in stepper and belt drive system with like a 5:1 reduction. What I found was the reduction was no where near enough and the motor was not nearly strong enough. I eventually added mor reduction to the point I am at now, which is 17:1. While this works.....kinda, I still have movement of my part when putting the bit into the wood. I carve gunstocks and set my carving system up a little differently that may be inducing some of this probelm. I offset my center to reduce the amount of high dollar wood I have to waste on the blank. Your centers will of course be on center and therefore won't be a factor, but the sheer size of the part tells me that you are going to need a hoss of motor to do what you need.
There is a fella in Dallas you need to get in touch with but I have tried his email and his url both of which seems to be dead. Masterwerkes was the company name and his name is Phill Pittman. Mucho talented man and I think he posted a long time ago here on the zone that he built an a a axis using a car differential! I'm getting a vision of Tim the Tool Man Taylor here with MORE POWER!!!
Mike