I've worked out the layout for the Z Axis ballscrew, using option 1 above.
This will require making a new threaded fastening collar, as the existing one is too hard to access once the mounting plate and ballscrew pulley are attached. The thread is metric with 1mm pitch and ~43mm on the major diameter. It is easier to cut the thread on the lathe and check while the mill is still dissembled so this is next of the list of jobs before I clean and reassemble the Z carriage.
The 45mm diam 1214 easy machining steel round bar arrived today so this is the first job for the weekend.