I CNC'd my J head BP and drive both the knee and Quill. Normally I only cut with the Quill drive as it's a little faster but if I have a deep hole to drill the 4" Quill travel sometimes in not enough.
Hint: That rusty looking plate below the pulley and belt and above the quill window can be duplicated but with a longer front section. Make a ballscrew thrust bearing mount and attach it the the bottom of that longer plate. Above or below that put a timing pulley.
Then remove the block that attached to the quill through the slot and replace it with a new larger block that captures the ball screw nut. Add a lower lateral support bearing and a stepper and you are done.