If I add an ambi, I add a stop on the port side of the frame. John Harrison explains it better than I can.
The right grip panel makes a decent stop.The thumb shelf on the right side must be fit so it bears either against the top of the right hand grip panel or must have a stop stud installed into the frame for the shelf to bear against. The joint must be a tight fit and the flat leg must not be sloppy under the grip panel. One way or another, you have to set it up so that when the lefty disengages his safety and then rides the top of the shelf, the force is transmitted to something that stays on the right hand side of the frame, rather than transferring through to the other side of the gun. The King's ambi uses a special hammer pin to retain the right side ambi. It's constructed with a dovetail on the right hand end of the hammer pin that engages a matching groove in the backside of the right hand half of the safety, keeping the safety from moving out from the frame.