No offense intended Historic....but do not cut slits in your fender lips mxmattd, there is no need for it on a Mustang, Shelby didn't cut it and I have not found a reason to since the lips roll quite well with a hammer and dolley/sandbag. You can trim the depth of the lip if you like a little to make it easier and cleaner especially in the back....there are two panel overlapped to make the rear lip and one is usually longer than the other so you can grind until they are the same depth....that works well and makes rolling a little easier.
The secret to rolling with a hammer is to go really slow and just roll the lip a little at a time. The metal will stretch well withouth getting damaged if you go in small steps, in fact once you reach the breakover part it is really easy.