Also think about how you will bolt it all together, you'll struggle to get the bottom Z axis bearings on as the bolt holes will be covered at the back so you'll have to take it all apart if you need to gt at them. If you extend the top Z axis palte back you can machine into it the adjusting slots you need and also it tucks the stepper down out the way and lowers the height of the Z axis.

Also don't forget that steel box section comes with rounded corners not nice square ones as you've drawn so you'll possibly need larger section (60 x 60) to be able to bolt the bearings to? I'd also extend the small plate you've got on the back of the Y axis for the floating bearing (small) mounting to provide a bit more stiffness for the Y axis, that way you'll not need any diagonal bracing.

If you're not sure what I mean attach the sketchup file (if that's what you're using?) and I'll show you what I mean.