I'm not affiliated with G&H, but I have spent a lot time there (buying, selling, trading, advising, etc). Paul and Bill are the only smiths that I trust tweaking an M-S or Brno. They've handled several major projects as well.

The Pachmayr rings detach from the base by turning the silver retracting knob in the center. That way you can change scopes easily, by having additional upper halves. If you cut the bar, you will be relying the tube alone to provide stability against the pressure of the male and female cones that hold the rings in place when the retracting screw is tightened. You kinda need to have the mount and rings in front of you understand how it works (or a copy of the instructions).