Any thoughts on an under slung gantry? I'm just thinking if i'm using 200 or 250mm box section and mounting to the top of the gantry then the Z axis is effectively 200mm + rail / cartridge height before its even descended at all.

It might be a little tricky to mount the rail, i'd have to jack it up off the bed I suppose. The gantry could be epoxy leveled upside down.