[bobbader]

You said it’s a repro gauge. Does it use a neoprene gasket or cork gasket? I prefer the cork gasket with a thin coat of grease or Vaseline. It compresses better than the neoprene gasket. And, you have the brass slip rings on the outside of the gauge body rather than under the gauge body? It sounds like, for whatever reason the gauge body is not seating into the tank flange enough to allow the outer nut (ring) to thread into the flange. If you have the outer nut from the old gauge you are replacing, give it a try just for fit. Some of the repro outer nuts have an undercut at the end of the threads so the nut can be tightened into the flange more. In this case, the undercut may be keeping the nut from getting started with the gauge in place.