[twindman]

I have a box the height of the bed rails and full width - no gap front or sides. I carry 2 sets of golf clubs and some truck things like oil, anti-freeze, etc. I almost ran out of fuel in SW Wyoming the first year - had less than a gallon left. We had traveled about 230-240 miles with no stations - ulp!!! First thing I did when I got home was add a 52 gallon tank under the truck. I didn't realize that the truck was near weight limit, and the extra 26 gallons of fuel adds about 300-400 lbs. If I had to do over, I think I would opt for a fuel tank in the back - probably smaller than 26 gallons. I don't know if they make a combo toolbox/fuel tank or not but that would be ideal.
Oh, the tool box is water proof and works great as far as that is concerned. So some things to think about.