Robin's usual suggestion is quite common on milling machines, including mine. It makes sense so long as the bed is very strong, though it's not normally seen on extremely high precision machines as the weight of the overhanging will cause some deflection, but that's hardly a concern to us.

Careful with the bearing spacing you've chosen for X - with a single ballscrew either the bearing spacing should be increased or the rail spacing decreased, else the stiffness of the X-axis is reduced when Y is towards either extreme.

By the way, when it comes to aligning the rails and bearings, I have a quite high precision straight edge and square now, so you're welcome to borrow them.