Dash board teepee

You are correct. The voltage limiter in with the thermal gauge was considered a lot of heat in that can plus that stud does get warm. There is a larger square hole under there. By 67 model they fingered out this was overkill. The warm stud got a bakelite cap similar shape to a household wire nut. No square hole in the board.
Please know that you don't need to open your fuel gauge unless you want to look in it. Simply lift it from the cast metal housing and isolate its backside from ground. A small piece of electric tape works. You can place a outboard regulator anywhere. Move the 12 volt wire from harness connector to the new regulator. Attach its 5 volt output to any thermal Guage. Copper trace links them all. Happy moparing.