I see a major problem here: inertia. You said the gantry weighs 260 pounds and you want to move it at 400ipm. That is a lot of energy. What does the table weigh? Unless the table is firmly secured to a very sturdy bench or attached to a large piece of steel/granite it will dance all over the place.
If you can get this sorted out it looks to be a very solid machine. Nicely drawn as well. What CAD program did you use?
Good luck, keep us posted,