Sounds like you're making a CNC router with ball bushings? Are you using monster linear shaft or splashing out on the pre-drilled? Have you considered cheap steel bars with ball races running up agin them and all bolted to a bit of box section? It isn't quite so off the shelf but it's certainly strong enough.

How big is it and how are you going to drive the gantry/head? Screws? racks?

Good luck anyhow :D