If you want a large capacity without loosing bed space, I would say your best bet would be to link up several smaller tanks. Such as 2 tanks mounted in the front part of the box in the "wasted" space on either side.
A little off the subject but, for you guys with on-board air, is it possible to run air tools off the system. Like a 1/2 impact gun for changing a tire on the side of the highway? Sure would make it a lot easier.