Quoted from the Ford SVO instructions that came with my FRPP C Springs..
Removal of the stock upper and lower insulators will lower the car an additional amount but will also increase harshness, road noise, and squeaks. If ride height is still too high after several hundred miles of spring "break-in", removing 1/2 coil from the spring will lower car approximately 1/2 - 3/4". Remove this material from the *bottom* of the *front* spring and the *top* of the *rear* spring if desired.
I just did this last night on my c springs, used a dremel and a heavy duty cutting blade. I cut almost exactly 1 coil off the top of the rear springs, so right at the end of the top pig tail i cut the coil underneath it. I kept the isolators on the top and bottom of the spring and it lowered it about 1/4 inch, but it definately looks better and the ride is the same.