I am going with a 20mm ballscrew on the Z axis.
The closed loop stepper will mount where the feed gearbox was previously located.
Ballscrew mounts and stepper motor mount were attached with socket screws. Was easy drilling cast iron, hardest part was keeping the drill perpendicular. Tapping was also quite easy using Tap Magic as a lubricant.
Timing belt pulleys will drive the ballscrew. A 20 tooth pulley on the motor will drive a 30 tooth pulley an the screw. This will give a little extra torque.
I was able to reinstall the chuck and tailstock to drill the pulleys.
I machined a block of aluminium to attach to the ballscrew mount, and this in turn attaches to the saddle with a 10mm socket screw to transfer linear motion from the ballscrew.
First test and the z axis moved perfectly!