I would use a shrinking hammer as a last resort unless you like your Metal "tenderized"-
In a pinch, go to your local Kitchen Shop and buy the thickest Stainless Steel Cooking Pan (with the shortest sides), bore a hole in the middle and mount it to your 6-8" Grinder-while wearing a Face Sheild, put it on the Metal to be shrank and place it on the Metal-turn on the Grinder, and let it run against the Metal for about 30 seconds or so. When you turn it off, leave it on the Metal until it has stopped-
Then, have a bucket of water with a rag in it nearby, and squeeze the water out of the rag and place it on the Metal-it should steam (or you didn't leave it on there long enough)-do this unitl your get your desired shape. Don't run it until the Metal turns blue, that's too hot!
Using a Torch is an art, a shrinking Disc will always ride on the highest part (the stretched part) and it smooth controllably-it will make you feel like a metal master-
Better yet, just buy one of these (this is the one I have and it is the original):