As with any project the right tool will make any job easier.
Since I am the one that rolled the fenders above let me shed some light on the process. The tool uses a polyurethane roller to effectively shape the fender. I rolled it with three angles to get a smooth transition without cracking the paint. The amount of metal stretch involved was small and did not exceed the elasticity of the paint.
If you want to stretch the fenders wider you need a different approach. As Sherwood mentioned a hammer and dolly technique works great. Not really for the faint of heart and body and paint work will be required when finished. Here is my work on the ST project years ago.
http://forums.pelicanparts.com/showthread.php?t=121226&highlight=roll+ST
The other option that will leave a smoother surface and reduce the overall time is a pneumatic plannishing hammer. there are some small handheld ones that have a 12" throat and are made specifically for such modifications. I have one but prefer to do the initial shaping by hand. The ability to direct the hammer blow to a specific angle, when used appropriately, is more important for the final shape. The use the plannishing hammer for final smoothing. It as this point where multiple small blows work well.
Best of luck on your project.