I had assumed that your previous drawings with the spindle back-plate were making it shorter so we could see past it, but your latest drawings seem to continue the theme. Are you genuinely planning to have the top of the linear rails for the Z axis hanging in mid air like that? Seems like quite a flimsy design decision for the minimal cost of some extra aluminium plate.

I'd also doubt that the ballscrew needs to be that long - but I gather you've already purchased it?