[dsraven]

in the bed is nice and all. easy if you don't have fab skills. I havenever had one there because it would be a pain for me.
-you can't haul anything when you need gas
-you gotta jump in the bed to fill
-you may spill fuel on your nice finished bed boards

when thinking about the filler location think about fill and vent. to fill the tank it has to vent the same amount of air easily or it will burp on you or take forever to fill. the filler needs to be higher than the tank so it doesn't try to leak when you go up hills or around corners. the filler needs to be as straight a shot as possible down to the tank. don't forget to use a filler ground strap to handle the static charge.
there are filler cap/door assemblies available online that bolt to the fender if you wanted a weld free unit. try to keep the filler high on the fender if you do that, less chance of fill issues. look for a flat spot in the sheet metal. the other thing to do is use the same contour of hole as the cab has, like copy the hole from the cab, and then use the rubber and filler neck from the old set up. there is one on the forum somewhere that did that,can't remember who though.