Because gas welding requires no grinding, no electricity, less chance of starting a fire, and is easier to hammer and dolly. Gas welding can be as fast as any other method when you take grinding and metal-finishing into account. There is no need to do a little bit at a time as there is with...
Here is an example of some work I have done with a shrinker and stretcher. A custom steel t-top roof for a Karmann Ghia. When shrinking a tight radius on a flange that is more than 3/4" or so it is sometimes better to cut the flange down to 1/2", shrink it until you have our curve right, then...
Kent White sells stippled jaws for shrinkers. They mark the metal far less and work great. His web site:
www.tinmantech.com
Spend some time browsing. He is a very interesting and talented guy.
John www.ghiaspecialties.com
You have already gotten some good advice. If you are interested in a simple oil can fix, I have posted something I did a while back on youtube that might be of help (second video):
http://www.youtube.com/profile?user=GullWingInn&view=videos
Torch shrinking is not as controllable a shrinking...