The 3AJ carries 22 useful litres as it is giving you a x country range of about 270 - 330 km , Which is enough for most trips. I suggest you look at carrying a jerry can mounted low on the engine bars for additional range. There is an argument that if you are in a remote are so remote that fuel is over 200km away that you should separate you supplies into 2 or more parts, The logic is that if your bike goes down and splits the tank, nearly happened to my wife’s bike in Morocco. see pic. that you only lose part of you supply and can dury rig up a supply from the jerry can to get you to safety.
Chris Scotts book show some good pics of jerry can mountings though for all but the most demanding routes I would put it with a lock lid on the pillion seat.
You could get your tank extended but you will introduce weaknesses in both the construction and mountings.
Dented tank pic: it could have been worse if it had hit the seam we could have had a seriously leaky tank.