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.
That toolbox looks like it would work great!
Maybe the lower half could house the water bladder and upper half could serve another role? Recovery gear/first aid kit/etc.
there are several things i like about separate water cans/bladders that trump a large single tank, IMO:
-if something punches a hole in one, you only lose a percentage of your water.
- the smaller containers can be carried to the source of the water easily to be filled
-bladders can be stowed in a fraction of their filled size once emptied.
-if bacteria is present in one container it doesn't foul the whole supply.
Just a few thoughts.