What about putting a fuel tank where the spare tire is now? Can't you get one from another duece and mount it on the other side of the truck?
You mean like this?
Rough details of what I had to do in order to make this work.
1. Source a second tank with pump.
2. Source a set of fuel tank hangers.
3. Find a fuel selector valve from an M818 or other 5 ton with 2 tanks.
4. Find a relay and switch
5. find a second fuel tank level gauge.
6. source misc hardware, hoses, fittings copper line, and nuts/bolts for the setup
1st step, take down the spare tire hanger and loosen hardware on the inside of the frame rails where the hoses and wiring are so you don't cut them when drilling.
2nd, mark the hangers and center punch then drill the holes.
3rd, hang the fuel tank hangers and tank.
4th Fab up a new vent line and tee it into the existing vent line. If you split your vents before you should have an input already. This is iirc, 1/16's line (small stuff).
5th install the fuel valve after attaching all the usual hoses.
6th install the fuel gauge in the spot to the left of the existing gauge, add a one way switch on the dash to control the relay.
7. install the relay in a useful spot. I elected to install an aluminum box on the bed near the existing tank so I didn't have to hack the wiring hardness to find the correct wire. This way power runs to the tank but is diverted to go to the box with the relay control wire from the dash energizing the relay to select the left side pump. I'll otherwise power the right side pump when the relay coil is unenergized. Thus it fails to using the right side tank if there's a relay or switch problem.
8. plumb up the tank and fuel lines to the fuel selector valve.
9. test it and now you have 110 gallons of fuel good for ~1000 miles on a good day.