I was also surprised how much heat the paint tolerated. I was prepared to have to strip the engine if the paint was damaged but it was fine.
The first time I tried this I didn't get it hot enough and did see it just beginning to crack. I heated it more restored the correct curve with the crack barely noticable. With more heat on subsequent jobs, no sign of cracks.
Zamac melts around 725F. I don't have any way to actually measure the temperature of the metal, one of those IR thermometers from Harbor Freight would be handy, but do know there is a point when the metal becomes very plastic and moves easily. You have to be careful not to mar the surface it can become so soft. Avoid tools with sharp edges.
Pete