First - do the math, consider HOW MANY more gallons of capacity you'll get per dollar spent on the new tank...you WONT beat an in bed tank price per gallon.
I put an ATTA Tank in my 2004 F350 (long bed) -
Aluminum Tank & Truck Accessories (aka ATTA) makes several shapes & styles.
I installed a 50g (in bed) tank w/ their
Freedom Fill (automated pump) system - The Freedom Fill, will pump fuel from the aux tank when the factory tank reaches 1/4 full. The pumping automatically stops when the factory tank reacts 3/4 full and the cycle continues off/on until the aux tank is empty.
The atta tank (when used with the freedom fill) provides a separate fuel level gauge for the aux tank.
My 50g tank takes up a shallow 10 inches of bed depth - even with the tank, I still have more bed space than a short bed would have.
Aluminum ATTA tanks have baffles (reducing sloshing) and won't rust (like RDS / steel tanks).
Here's some pictures to give you an idea of how little the space consumed is -