Yes Hammer on Dolley will stretch the metal, Think about it your squeezing the metal out thinner between the Dolly and Hammer its getting squished with each strike of hammer blow so its getting thinner and its stretching the metal.
If done enough in one area of a Panel it can cause an Oil can during a repair like this.
Some is gonna be unavoidable the metal just wont unlock with out doing it but Go slow concentrate on Pushing hard with the Dolley and tapping metal off Dolley as much as possible. Going Hammer on Dolley keep to a Min as you can it will be required to get panel finished. and Like I said you may need a shrinking Disk.
https://www.eastwood.com/wolfe-s-easy-s ... bjEALw_wcB

That puller attachment was store bought
https://www.nationaltoolwarehouse.com/U ... RnEALw_wcB

Here is a few others that are handy to have with a Stud welder.
https://www.jbtools.com/hs-auto-shot-21 ... PCEALw_wcB

https://www.jbtools.com/hs-auto-shot-10 ... lding-tip/

https://www.nationaltoolwarehouse.com/U ... I6EALw_wcB

https://www.ebay.com/itm/203229820754?c ... 9fEALw_wcB