I just broke down and bought a diamond-plate aluminum tool box. It has 7 cubic feet of storage space. It was made for a mini truck but fit nicely in the back. The reason I mounted it off-center is because I mounted it to the floor using the old bolts for the spare tire holder thingy (which was not there when I bought the truck anyway). There were 4 bolts and they were on the left side of the floor back there and I wouldn't have been able to use them had I mounted the box dead-center.
I knew I would find a good use for those old fender badges I found on eBay!
Dontoe after seeing how good yours looked I pretty much knew that's the direction I was going to go.