1967/74 F100/350's have a manually operated fuel tank selector valve on the floor adjacent to the drivers side of the seat.

1967/72 only: To the left of the steering column, below the dash, mounted to a bracket is a toggle switch that only changes the dash fuel gauge from the in-cab tank to the auxillary tank..

The switch bezel is market L FUEL TANK R
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1973/74 F100/350: The switch is located in the heater control panel.

1975/79 F100/350's have a electric fuel tank selector valve and solenoid assy (D5TZ-9189-B), mounts on the frame rail.

The switch is located in the heater control panel, switches the gauges and the fuel tanks.