I'll look around and see if I can find one, even 13 extra gallons would be nice. I travel for work and would love to stop less on the interstate
The only tanks people have been able to find are rusted out as far as actual Ranger auxiliary tanks. Some have used Bronco II tanks instead but they are a bit too big and hang down below the frame. They also hold more than 13 gallons but I forget the capacity. You'll also need to figure out how and where to put the filler neck and access to it.
The Aussy tanks you speak of, I've seen pictures of them. They arch up over the driveshaft and take up the little remaining space between the driveshaft and the exhaust. Most Rangers outside of the North America tend to be diesel. So, those tanks may not be compatible with the North American Ranger. Plus diesel is less flammable than gasoline. So, if you can make it work, it will need more heat shielding to make sure the exhaust isn't a problem. This is assuming you can get one shipped to the states to begin with.
The best option we have found so far is to use a certified tank, whether it be the OG aux tank, the Bronco II tank, or a prefabbed fuel tank from other sources. Then you will need to figure out the wiring and switching to make the system work. The other option is to mount a tank in the bed and fill the existing fuel tank when needed or use Jerry cans.
Long distance travel without needing to refuel has just not been a priority in the US like it has been in Australia or Africa. So, there is no demand. We wouldn't be using gas engines either. Diesel is more fuel efficient, thus why all the overlanding vehicles elsewhere in the world use them.
If you figure something out, let us know. Some of us would like to have more fuel on board. We just haven't figured out a way to do it without either mounting a tank in the bed or major fabrication work to make a barely workable Bronco II tank work. As far as I know, no one has used any other tank from a racing application or other vehicle and made it work either.