Ok, I'm a bit confused. You keep saying fixed gantry but then you show a bed with rails on the side.

Is this the bit that will be moving? If so I would have said that it is not the best way to do it. Moving beds should have the rails underneath and at approx. third points or a quater in.

That way the rails are supporting the load better and are closer together so you can get away with one ballscrew.

As you have drawn it the bed can bend in the middle under load.