That's fairly minor damage. A skilled man can easily render that invisible in an hour needing no filler. That said, easily is determined by skill and experience. If you have less, you might have to take more time and care. Ideally you would weld pins along the crease and gradually pull the dent out with a slide hammer as already noted. A pin welder isn't exactly cheap, but you may be able to get somebody at a body shop to do that bit for pretty cheap as it will only take a few minutes. I would not expect much stretching as more than likely the tank simply distorted very slightly to accommodate the dent, and once pulled back it likely will simply return to its former shape. After that, a hammer and dolly will dress it out, but you'd have to weld one on to a stick to hold it properly inside the tank - again not something most guys will be able to do easily. That is the only way I know if you want to keep a steel finish. However, if you are going to paint, you can simply pull the dent out close and use polyester filler (Bondo) to smooth things out. There is absolutely no downside to using polyester filler. If done properly, it will never shrink, crack separate or fall out and is very easy for inexperienced hands to deal with. Like anything, performance depends on the quality of application. Lead is bomb proof, but has its own problems, is vastly more difficult to achieve the same results with and you are far more likely to have long term problems from flux contamination than any problems you may get with plastic filler. I suggest you avoid any form of pressurizing your tank and/or heating it up with a torch. Both can be used under certain circumstances to good effect but this is not one of them, or at the least is completely unnecessary and can make a great deal of trouble if you don't know what your doing.