work the dent through the filler cap. The rod needs a point of leverage to push against as you push the tip (always use round stock bars, never flat) into the dent. Start at the top or bottom of the dent, never in the middle. Work gently and slowly down the crease, raising the edges as you go. As you approach the "middle third" of that crease's length, switch to working form the other end (top down first, bottom up second, then the middle). This way you won't drive the crease deeper into the middle portion, but coax the metal up.
You can buy a PDR tool from an online supply house. Get a 3/8" tool, probably about 2' long (trust me, you'll need leverage) and cover the tip of the rod with Duct tape to reduce the sharpness of the point. If the tip is too acute, it has the potential to dimple the metal internally and leave small blemishes.
The one thing you'll want is a light source laid along the tank to show you the shadow of the dent. As you repair the dent, the shadow disappears, thus the metal is flush. Its how body guys see dents and dings. Dents distort light and create shadows that you can detect. (I know you probably know this, but it's an explanation for others who may follow your progress).
Lastly, leave the tank mounted securely on the bike, bike strapped to a lift. You'll be leaning on the rod pretty good, and need the counter-weight. This will enable you to drive the metal without chasing it all over the shop. Just keep an eye on the filer neck that you don't push so hard against it to deform the metal around it.
I did a tank for Stev-o that was a survivor. Couldn't get it perfect, but among the flaws was the fill neck was depressed. Used a puller to jerk it up back to stock level. The neck came out pretty darn well that I'm not sure he ever knew.
