Ideally, you want to keep the spindle as close to the gantry as possible, especially with a long gantry like that. It acts as a lever, and the farther away it is, the more twisting force it applies to the gantry. Adding the carriages will make it more rigid when the spindle is at either end, but you still may get some twisting of the gantry with the spindle in the center.
Also, you may be disappointed with the electronics kit you purchased. Those motors are much to large for that drive and power supply, and probably will not perform well.