Alright if we are sure the cluster has good continuity to ground, I'm wondering if there is a prob on the supply side, (that fire from the dash lights that get turned on when you turn the headlights on) is somehow bleeding over & full fielding the gas/temp gauges (giving them full 12V) & making them peg. I hate to do it but I would pull the dash & grab your meter. Actually first I would pull the HL bakelite connector off of the HL switch & pull the wire/terminal for the dash lights then replace the connector on the HL switch & turn the lights on & see if the prob goes away. EDIT female packard 56 terminals have a tang on the back side & go in the back side (where the wire is) with a mini eyeglass screwdriver & depress it & when it is flattened enough the the wire/terminal will EASILY pull out the back side & if not dont force it then go back in with the screwdriver