Quote
if you could illustrate to me a way of centering the spindle without giving up Z, i'm all ears.
Unquote
You could extend your X axis frame on the top to be outside the tubular box frame. The vertical plates that connect X and Y can then be placed on the outside of the tubular box frame. The connection between these plates can then substitute for the X axis framework above the tubular box frame.
Z is moved back and is then free of obstuction to travel upwards.
This modification would mean that you cannot add another vertical to the long sides of the tubular box frame as it would obstruct X axis travel. Perhaps just strengthening the existing design of the box frame will be sufficient.
Placement of a single ballscrew for X might have you back to the drawing board.
cheers,
Rod
Perth, Western Australia