Dave that base frame is way to weak and when you get a gantry on then the inertia will rock that frame like a Jelly. Needs plenty more bracing.

Regards the X axis (Which I call Y axis) then just use the same method for Y & Z axis.? No point making the other Axis better than the X axis has the machines only has good has it's weakist link.

For a plasma machine then these rail methods will be fine has you only really need positional accuracy not ridgidity like a router/mill does,