The new, thinner, high strength steels are definitely harder to work with than the old stuff, but excellent results are still attainable.
As for your bike tank, you always want to place your welds in an area that you can get to on the backside. When custom building a new tank, you will place the welds where you can get to the backside of them with a post dolly or planishing hammer. Again, you have to be resourceful and make your own tooling at times, but don't let that stop you. Bike tanks are normally built out of heavier metal than automotive body panels, and coupled with the high crown shape of the tank, distortion can be held to a minimun if care is taken. You normally leave the bottom of the tank to weld in last, allowing access to the welds from the underside of the tank.