[dsraven]

I have seen a few modified to accept a filler on the rear fender. some nicely done, some farm/work truck look, some race car look with the bolt on flange around the outside, some that look like the stock cab filler except on the rear fender to look period correct. some have a door, some don't. the door ones are harder to get looking and fitting nice on an egg shaped surface. personal preference. personally I wouldn't put the filler in the box floor because I haul stuff. of course I would need to fill when there is something in the box, lol. also, the box floor sometimes means the filler hose needs to drag across the paint.
I remember as a kid at the filling station we had a guy with a 67-72 era chevy pick up and he had a rear tank with the filler behind the lic plate. when we seen it coming we would always try to get the other guy to run out and fill it up because it almost always burped back and got gas on the ground or your shoes. maybe some of the newer tanks have the anti siphon/roll over protection valve installed at the tank entrance of the tank? sometimes they are built into the tank filler hose and sometimes they are built right into the fill connection of the tank.