Not something I'd recommend unless your doing the machining yourself and have the rails in hand to take accurate measurements from, even then you would want to creep up on the final dimension slowly.

I certainly wouldn't farm the job out and expect it to come back a perfect fit.

However, it's not needed, all you need is one reference edge to butt the rail against.

If you really want the rails clamped against the ref edge then you can use a method often done on milling machines where you drill a series of holes running parallel to the rails offset by the width of a dowel pin, say 3mm, then using taper screws in the holes the taper pushes the pins against the rails which clamp the rails up to the reference edges.