I have one of these:
http://www.kshmarine.com/toolbox_tank_combos.htm
They custom make almost anything you want. My Ford had a switch that was plumbed into the fuel lines and a flick of the toggle and the AUX tank took over, gauge and all.
My Dodge has the same tank, but the switch doesnt work on a Dodge, so I plumbed into the filler neck of the main tank and push a button to refill the main from the AUX.
but topping off becomes a guessing game.