The rails are bolted to the box section, even in post #15. Earlier designs for a 1000mm gantry had them mounted on 90x45 ali extrusion which was bolted to a back plate.
I was hoping that tacking the sections together first and then do gradual passes would eliminate any heat distortion but if it's out of spec for the bearing before i even start, well that puts the mockers on that idea. Any idea how 90x45 ali extrusion would cope over 2m?

I would have liked to have the ballscrew mount on the Z axis upright so I would have to remove the middle box section and bolt to a 20mm back plate but then then is no room for the ballscrew mount unless the width of Y is 200mm+ to accomodate the mount as i feel it best to surround the Y bearings. Y ballscrew is currently 100mm off centre from the Y bearings.