When we had our stock bed we just went to Lowes and got a box. It had a few inches of play on sides but not much. jfyi when we put hauler bed on everything we had in box fit in bed storage boxes with room to spare. We can see to back up now too

It isn't that big but Wal-Mart has a bin about 36x17x17 for 40 bucks or so. We had one until we got your aux tank installed and used a bike cable to lock it to the truck. Swapped it for a couple off locking Rubbermaid bins that will fit behind the hitch. They were 45ish each. 

I got one of these at a local Four Wheeler Parts Store. I wanted bigger but they only had one in stock which actually turned out to work great. I don't remember the demensions but it sounds like what you need.

www.deezee.com/products/399/Truck_Tool_Boxes/Truck_Tool_Box_Chests/Red_Label_Utility_Chests_-_Black.html

I mounted mine in the middle so there are gaps on either side that are just big enough to stack 2X4s I use as leveling blocks. Plus it allows me to carry 8ft long pieces of pipe or wood inside of the bed if needed.

The lid of the box sits just below the side rails of my pickup bed.

The metal used on the toolbox is a little flimsy but I'm sure that's why the cost is low compared to others.

One caution, the black is sprayed over the shiny diamond steel. The black will peel off if you use a pressure washer too close.

-- Edited by The Bear II on Thursday 22nd of January 2015 11:36:20 AM

We looked at the following websites when we ordered 2 different tool boxes, including the smaller partner box that fits between the hitch and tailgate:

www.800toolbox.com/5thwheel/5thwheel.htm

www.pickupspecialties.com/toolboxmain.htm

We were able to find 2 tool boxes that fit within our specifications, allowing room in the truck bed for the hitch and the generator.

It's in my signature. An ER model by CM a bed manufacturer. I got $400 for stock bed at salvage yard. Mine has trough and it's 18" wide. Don't know of any hard hitch that will fit in it. We got an air ride hitch from Hitch Crafters. Big Tex Trailer World is where we purchased and had it installed. You can likely find it for less money but all we had to do is drop off and pick up. You get a lot more use out of a bed. Even getting propane simple. Ever secured a 30lb propane bottle in a stock bed? We just strap it to rack. If you go this route weld some extra steel under trough for hitch before bolting it down. Be an easy job then. Little difficult with bed on truck.