[wingnut60]

Another option is a tank from RDS Aluminum from Northern Tool. I have a 60 gal tank/toolbox in my 450--had it in my previous 350--and it gravity feeds into the truck filler line using a check valve also purchased from N. Tool. It will not overfill the truck tank and it just keeps the tank topped off as I drive. It caused some CEL issues on the '07 350--has not happened on the '09 450. The Miles to Empty stays at 415-490 until the aux tank empties, then the MTE starts downward.
I would not be without this extra tank option--I have easily paid for the original cost from fuel savings being able to check prices on gasbuddy and planning my purchases. 20cents/gal on 80-90 gal adds up.