It looks like the cab tank is connected to the rear tank ,does the hose go from the petcock to the rear tank.if so then I see the rear tank has wires for a gauge so it appears the gauge runs off the rear tank .It looks like it was originally set up to have the tanks separate with the switch to the solenoid that looks disconnected .It maybe a safety concern the way it is now as if both tanks were filled the cab tank is gravity fed to the rear and if you parked on a hill there could be leakage.You could start by taking the cab tank out and clean anD put it back in ,then make up a line like the original one that goes from the top of the tank that is closed off to the original hole in the bottom of the floor to the line going to the fuel pump.The 54 tanks also have the same petcock at the bottom and the gauge looks like my 54 .The wire for the cab tank comes from under the tank and a ground wire from one of the top screws on the sender to the brace at the back of the cab.
