You want the X-axis rails attached directly to the bed, not the bearing blocks. It'll save material and keep the bearings (i.e the support) in the same place in relation to the spindle regardless of the X-axis position. The same goes for Y - orient the rails like on the machine Jazz posted.

You've mentioned using 25mm ballscrews, but it's a bit early to be deciding that yet. On a router we'd say use 16mm screws, as the critical speed isn't a limiting factor for this size machine. However on a milling machine it can be worth using bigger ballscrews if the increased stiffness is beneficial - but you can only know that when the frame (etc) is designed.

Quote Originally Posted by Blackrat View Post
pouring aluminium into the tube will just make a loose slug, the ally wont bind to the column
Yep, and it won't do a lot for damping either, which is the purpose of the epoxy...