Jerry, I have just test flown my setup as you are wanting to do. I have two G-38's on my Zirolli C-47 and of course the shaft extends out the rear as yours. I turned two aluminum discs on the lathe and they are in turn screwed onto the rear shaft of the engine. The sensor was mounted on a plywood "mount" that was in turn fastened to the firewall. Works like a champ and keeps the sensor away from the magneto up front. I have had no problems and range checks are as good as usual. Wayne Parrish