I would consider moving the ballscrews just outboard of the ends of the gantry. This allows a more rigid fixing of gantry to carriage feet. If the ballscrew mounts are on outrigger brackets welded to the outside of the frame tubes, mounting pads for the ballscrew bearings could be machined at the same setting as the frame tubes. There is no benefit in having the ballscrews over the slides as a gantry driven at both ends is balanced anyway. It's not like the gantry ballscrew which is more difficult to fit between the linear rails.

The idea of being able to machine the top of the long rails is so far from reality for most of us that I doubt if many have designed for that! I'm in the "if I can't machine it, build in adjustment" camp...