Thanks for the reply Jonathan, and I will space the X axis rails further apart.
So if I’ve got this right, this process should work.
  1. Position one of the X axis rails to the frame as straight and parallel to the frame as possible, using a taught wire or other method.
  2. Using a transfer punch mark the fixing positions of the rail support to frame.
  3. Drill, tap and mount rail, and recheck straightness.
  4. Roughly position second rail and mount a jig/fixture to the bearing mounting blocks on both rails, and slide back and forth adjusting the second rail until motion is smooth.
  5. Mark positions of fixing positions onto frame and drill, tap and mount second rail.
Up to this point I think I’m Ok. The problem I see is due to the FK and FF supports for the ballscrew. These don’t seem to allow for any adjustment as the fixing positions would need to be made in the frame prior to fitting?
Maybe I could fix the FK support directly to the frame and amend the design to allow for an adjustable plate at the FF end.
Would I need to have the table made and fitted at this point to attach everything to it?
How would you ensure the rails were level vertically in the Z axis as this could result in the rails being like a wave. Should you shims or use some sort of leveling compound and how would you measure it?

Jim