I found what is (in my situation) the perfect use for those empty cavities. The one on the passenger side anyway. With a little trimming an Oasis 12v 8cfm air compressor will fit in there. I'll attach a pic. It's held on with cable ties at the moment, but threaded rods with lock nuts will hold it when I'm done. Also, you don't have to remove nearly as much sheet metal as I did, but it made fitting the comp easier. Since the comp needs a close battery source for full pumping capacity I installed a new Optima yellow under the body. And a 2-gallon air tank fits nicely between the muffler and the body. In all, a complete air solution with no lost storage space! Once you have on-board air it becomes indispensable. Eg: refilling my tires in 8 secs, power washing the truck, filling an air mattres, running air tools, cleaning the truck out. Hmm, can't post the pic. Anyone tell me how I can?