That tank is designed for the floor board and to go over the driveline hump it looks like.. I may work for what i'm trying to do, but more shooting for something like this... http://www.pickupspecialties.com/toolboxes/Trail_FX/T83-Wheel-Well-Toolbox-10.jpg
The bladder idea would probably work! As of late lastnight I was thinking if maybe I should try to relocate lower to along the frame somewhere.... My crew cab long bed has a lot of underbody space.