I moved my tank to the rear primarily to get rid of the gas smell. Even with new seals and gaskets it almost always smelled in the cab, especially in warm weather with a full tank, since the vented cap was right next to the drivers window.
One way to eliminate the floor fill is to mount it in the side wall inside the bed with the fill pipe under the (stepside) fender as shown on the 6th entry of the following link. Not sure if this would work on a fleetside or not. I made a mistake on the inlet to the tank so it didn't work as well as planned, but I'm convinced the concept is sound. Just another option to consider when deciding how to fill a rear mounted tank.
http://67-72chevytrucks.com/vboard/showthread.php?t=390279
