This is where I put mine in my old '96. I just screwed the mount right to the transmission hump with some short self drilling screws. I plan to do something similar in the '07 but turn the extinguisher sideways across the front of my center console.
I wedged mine between the toolbox and the side of the bed. I would be afraid to keep one in the truck incase it went off. I had one go off in the bed and they make a mess.
I have a chest tool box sitting on my L fuel tank in the bed, mine are mounted to the side of the toolbox below sight line. I have one mounted on each side of the box.