What about one of those plastic fuel tanks that are wide but thin. Something to lay on the rear floor, or tops of the inner guards, or behind the front seats. Perhaps they can gravity feed into the OE tank.
http://www.dolium.com.au/Poly_Water_Tanks.html
Alternatively you possibly can get one made. I have had several tanks made from folded and welded plastic sheets. Boat and caravan builders do a bit of that fabrication, though mostly for water.
I had a 107 pick up I sold to a bloke who drove it around Aus. It had new tubed canvas tires and did the 20K lap without issues. The brakes on that had been boosted with a generic after market booster mounted under the body, something widely available for parts if needed. Also had two coils mounted on the inner guard in front of the exhaust. The vehicle got caught somewhere with a cooked coil put down to heat off the exhaust. So the re fit was 2 for a quick swap over and an easily mounted spare I guess all infant of the heat source.
I believe the 107 was largely trouble free on its trip. All stock gear with the 2lt donk, coils and fuel pump. The owner put an overdrive in it to help with the large distances. I think it gave him an extra 10km/k cruise speed.
Good on you for have a dream, good luck and happy travels to you and the family.