I ran a 64 Nova tank in my truck right behind the rear axle between the rails. You can barely see it in this photo. I then made a filler door using a hinge from a 64 Impala as I remember.
Of course the wrecking yard isn't full of 64 Impalas and Novas so I am not saying you should do exactly this. But you get the idea. Your frame is wider so you could easily fit a lot of tanks in there that are at the wrecking yard. Personally I don't like the filler in the bed floor just because I used my truck, not to haul gravel but camping and stuff and I didn't want to have an access problem with the filler. The door in the fender worked perfect. And with a couple of pieces of exhaust tubing you can put the filler anywhere you want just about.