I'm attaching the motor just to the side of the gantry on a sliding mount thingy....much like I have on the Y-axis (see by build log). I'll mill a slot in the gantry side for the belt to pass through. The outer rings of the tapered roller bearings will be held in some 20mm aluminium plate, with bolts between the plates to apply preload to the bearings.
It'll be a while until I make that.

Note how I've made the delrin (might use brass...) nut in 2 parts, squashed together to eliminate backlash. That's the theory anyway.