I got around to fitting the new motor on to the Denford Orac last night. I had to remove the register on the motor so that it would fit properly, but once that was done, the pulley fit straight on the shaft and the motor bolted straight up the standard motor mounting position.
The shaft is not as long as the shaft on the old motor, but the amount of key way that is engaged is larger, so everything should be fine. Now just need to do some wiring up again.
I also ordered a transformer to make a decent linear power supply for the stepper motor drivers. I ordered a 1000VA 2x42V toroidal transformer from Airlink Transformers.