As some markets got aux tanks from the factory Toyota do a split filler for 80's (and some other models), with a knob you can pull out to direct fuel to the aux tank or leave pushed in to fill the main tank. Other than the knob it's like a normal filler from the users point of view. On a diesel you then just fit Toyota change over solenoids and run pipes to the aux tank and have a button on the dash to select which tank fuel is sucked from.