By gettin the worst of it close by supporting it from underneath with a round dolly and using the shrinking hammer directly over the dolly you will be able to shrink it somewhat. Once you have made a little progress take a propane torch and get a small area about the size of a quarter hot. Use the wet rag method as stated by denny. This will tighten the metal and allow you to work it again and again til you get it as good as you can. The disc is a good idea but if you've never done it you can burn thru the metal. If you get too frustrated and have had you're fill it is easier to smooth out a low spot (THUMP!!!!) Welding a piece in would create a nightmare for you since the heat would distort the heck out of it.
