Just a thought.
If you made the Z backplate wider, keeping the Y bearing blocks where they are due to short rails etc. you could then move the Z bearing blocks wider and fit a wider Z front plate.
This might give you the opportunity to mount the spindle in 3 positions, left, right, centre, not ideal but maybe useful ?
Or maybe you could then mount another device beside the spindle such as a laser head, 3D printer head ?