I'd U-bolt a couple of pieces of channel iron several inches long to the frame rails, join them with a piece of 1/2" or 5/8" plate, and mount the ball just below the level of the bed with a trap door to access it for coupling. It's simpler (and less expensive) than a flip ball and you will still have a flat floor in the bed when you're not pulling the trailer.
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!