I'd suggest a 3/4" plywood partition between the wheel wells and the tailgate, and about 10 milk jugs full of sand behind it. If you really want the load secured, run a couple of pieces of 2X4 forward against the front of the bed. When you're carrying a big load, it's just a matter of a few minutes' work to remove it. A tool box bolted to the front of the bed behind the cab with a few hundred pounds' worth of tools, chains, a jack, stands, and various other stuff was a pretty normal thing for me, but all my trucks earned their keep. They weren't Cowboy Cadillacs.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!