As above, while you can get away without a bearing on the floating end (you just end up with a lower critical speed which isn't generally a big deal on a Z-axis) you absolutely do not want to go with the way it looks like you've designed it there with the screw attached to the motor. Ideally you want a set of angular contact bearings on the fixed end.

Given the design you already have, it wouldn't be too much work to raise up the sides and then sit the gantry flat on the rails - that will give you much more rigidity than using the longer gantry sides, then just move the ball nuts to the side. Unfortunately I'm very limited in width and I couldn't get away with that design on my own machine given the work area I wanted, but it's definitely the way to go with a machine like this one.

what is the span on the gantry? how thick is the extrusion? looks to me like you'll get some fairly significant deflection fore-aft which may limit your surface finish and speeds/feeds in alu. I'd consider beefing that part up personally given the fairly decent looking span.