I am looking for someone to collaborate with me on developing a way to use the z axis with a traditional lever (drill press type) switch over from the mpg.  Someone who has experience with the mpg and Mach3 would be needed.  Contact via email to discuss further
nelsonk11@hotmail.com

Sounds to me like you just need to set the spindle lever up as your Z axis. pop a stepper and a belt drive on it and configure Mach3 to run with the MPG you have, and you should be good. With this said I think you'd be better off programming a drilling cycle for what you need doing, it will be more consistent, quicker and safer.

I suggest you consider something different. There is too much backlash in the rack and pinion.
Not an answer to your question, but been there done that and similar.

RICH