DO NOT CUT YOUR SPRINGS!!!!!
Have you seen a ricer budget slammed Civic go over a bump? They bounce for a block or two afterwards. They have cut springs. When you cut the springs you are changing the spring rate. For example, Ford Racing F5300-B springs have a 425 lb/in rate in the front. If one was to cut 1/4 of the spring out, you would effectively reduce the spring rate to 318 lb/in resulting in a "boingy" ride. I know you wouldn't cut that much out of it and you see guys like Chip Foose do this on shows like Overhaulin, however it is hardly a best practice.
If you don't like the ride height, I suggest selling those springs and buying a more aggressive lowing spring. Coil overs are nice, but expensive! Please for love of all Mustangs everywhere, don't cut your springs!