How big are the bolts for the rails? Will they fit in the profile that you've drawn? As Jazz says the profile is looking a bit small and weak, you could do with beefing them up a bit. It's looking like you're putting the Y axis ball screw on the front of the gantry, you'd be better putting it on the back and bringing your spindle nearer the front of the Y axis otherwise it could be quite a way off the front and a way in front of the X axis bearings. Are all the rails the same size?