There might be a way to insulate the gauge assembly from the dash with some creative use of paper or plastic gaskets, and apply the wire from the ignition switch to the part of the gauge that would normally be grounded to the dash. Then reverse the wires that would be attached to the sending unit and the insulated post where the ignition voltage would be applied. When I was driving an old Freightliner with a positive ground system, I wired up a radio that way, using a wooden box to keep the radio case from getting grounded and running the power wire to the case. Worked great!
Jerry
