If you get rid of the coupler and mount the nut right, I believe you can keep the motors in the "normal" location for the non-Acme thread rod driving motors. That's what they did on the i3 MK2. Worst case would be a printed spacer between the motor mount and the "chassis". I'm not sure if you could mount the nuts *below* the lower X arm, but you might be able to. That would make the required length of Acme rod a bit easier to find.