As for cutting out the mount for the fill neck of the bed and placing in on the other side, i was planning on doing that, it will look a little weird as its not going to hug the fender flare and it will open backwards (towards the rear of the truck).
You should be able to use a door from a left side tank, it doesn't have to be from a 1st gen Toyota to work. You also wouldn't have to have in the oem location. Depending on how the tank is set-up/filler neck location, you could likely install the gas door in front of the flare similar to this
Chevy S10, and it would open to the front.
If all of the other ideas fail/appear to be more work/money than they are worth, the 5gl. in bed idea isn't a bad one. You could mount one on each side, either in front of(better center of gravity) or in back of the wheel wells and fab up some panels w/a vent to lock/enclose them. Those areas are often dead space when hauling most things, and with them enclosed you wouldn't have to worry about gas getting on camping gear, groceries, non-refuse, etc. You could plumb them into the filler neck just above the tank, and either have them gravity fed, electric vale, petcock valve, or if they exist very low pressure one way valves. It shouldn't cost more than a couple of hundred to do, and you would almost double your range.
