I'm hard of understanding here. If you have a line from 1 tank down to another, the fuel will flow. And if the level of fuel is above the fill neck of the lower tank, or any vent, what will stop fuel from coming out the vent?
Just drinking bleach here; Does the pump in tank send more fuel to injector pump then the engine will use, then extra returned to tank? If so, put a tee in the supply line between tank and injector pump. (or filter) On the leg of the tee that goes to factory tank put a normally open solenoid valve. The side going to engine, no change. The other leg, a check valve strong enough to hold against factory pump pressure. Have the pump by add-on tank feed thru the check valve. A switch on dash cuts power to factory pump, closes the valve in supply line, turns on added pump, and a light near fuel gauge. Fuel would feed thru check valve to engine, and the extra returned to OEM tank. As you drive the fuel level shown on factory gauge would rise, near full, switch back to OEM function
