Hi, and welcome to the Zone!
I think that even with the rails supported on bolts every 10", there will be a lot of deflection, especially side-to-side. If you can't justify the cost of supported rails (I can't) then perhaps you can fab some with good ply on edge and side-to-side braces.
The top plate (wood?) is also going to flex a lot unless it is very well supported at the edges and in the middle. Have you considered a torsion box?
The X drive screws may pick up a lot of swarf where they are. If you went the torsion box route you could place some of the mechanicals on the sides to get them away from the cutting area.
Cheers!
Paul Rowntree
Vectric Gadgets, WarpDriver, StandingWave and Topo available at PaulRowntree.weebly.com