If you’re OK with keeping the tank in the cab and you don’t mind the noise an external pump makes, then yes the in cab tank can be used.
A (very pragmatic) friend of mine kept the tank behind the seat, but run a line from the bottom to an other tank between the rails in the back.
The return line also goes to the filler neck of the original tank - external pump somewhere in the back on the rails close to the new tank.
Many ways to skin a cat, IMHO not a bad solution if you want to keep the original filler, and almost twice the fuel capacity now.
Frank
