IMO the donut dolly or a heating tip on a dent puller/stud welder will give you a more consistent shrinking process for more controllable results. The shrinking disc and torch method are a bit more aggressive and thus somewhat more challenging to control the results. Here is a thread that details using these processes (except the torch) to fix sandblasting damage, or basically stretched metal, just as you have. It is a bit of a long read, but should help show the processes to fix your stretches..
Shrinking to repair sandblast damage
