Hi Matt,
Yes I did use the 3 outlet sending unit for the new tank.
One port was used to go to the fuel pump.
One port was connected to the unused port of the factory tank.
Last port was capped off.
The sending unit is a 90 ohm resistor, so I used a fuel gauge from Summit:
Fuel Gauge
Like Rory states, the switch will only run the pump while you have it depressed. This way, the pump will not overfill the stock tank if you forget to turn the pump off. I did that in my garage during testing, so I highly recommend that you use a momentary switch.
His idea of showing the fuel level as well is really a good idea. I had a custom console made up and added a second fuel gauge.
My idea of a 2nd tank was to safely carry 20 more gallons of fuel without the need to mount fuel cans and the need to stop and dump them in. (Sometimes fuel cans have a way of disappearing if they are not secured.) This way, I can "refuel" while driving down the road.
The only down fall has been at the fuel pumps. You will need to turn the truck around to fill both sides. Truck stops have pumps on both sides, but I prefer to stay away from those pumps. They are not friendly places.
Karl