We have this same cap but only ordered the storage boxes on one side...the drivers side. They just in about as far as the wheel well. We left the passenger side to be a full length window that swings out.
Makes loading and unloading things great and doesnt mean you have to climb into the bed to get something out of the back.
If I was to build a sleeping platform, we'd still have about the width of a double bed back there with the toolbox on just one side.
In a TACO, you'd easily have a single bed or more.