I suggest you calculate the force applied by those steppers on your leadscrews.Then visualise the likely effect on the position of the gantry beam.After which you may decide to add some triangulation to hold things in place.The gantry beam also seems very high for a machine with a 140mm Z travel and keeping it low will do a lot for the rigidity of the machine.It may also be useful to model on trays for drag chains as they are easier to integrate at an early stage.Good luck with the build.