Welcome Morgan, I'm in Prineville and work in Redmond. And I have a 76 F250 4x4 Hi-boy truck. My fuel valve is fixed so when it is pointing to the back of the cab: it is on the main cab tank, pointing at the drivers door; it is on the left tank and pointing at the passengers side; right tank. If your valve is under the cab on the drivers side; follow the lines and see which tank it is drawing from.