Ok thats good but bring the screw to the front, will be much stronger having the ball-nut close the Z Axis back plate.
Ok, thats no problem, i will do that. Is this to prevent racking etc

Have you got the 20mm steel plate.? If not I think you'll maybe change your mind when you see the price.!! It's not really needed either and I'd use ali for easy working.
No i haven't got any steel plate and a quick google has made me change my mind, but if i use ali how would I join this to the steel gantry?