Another thing you might consider is welding more studs on so that you don't pull as many times on each one. (does that make sense?) The ones that broke off could very well have been welded on properly but no matter how well its welded on, it will only take so many 'pulls'. Kinda like bending the paper clip - you can bend it back and forth a few times but it will reach a limit when it will break.
Practicing on a loose, unmounted fender will not work very well or give accurate results simply because the entire fender will move too much - got any dents on your truck??? got a hammer to make some??
![Wink :wink:](./images/smilies/icon_wink.gif)
Anyway, keep plugging away at it...............