I have used the Ford "C" springs on 4 Fox's. I love them. They are firm as shit. The fronts are a straight up 650lb and the rears are basically 5.0 progressive rate, that just lower the rear. They do drop the car quite a bit more than Ford says they do. Ford Motorsport used to sell the Reds as a match for the C springs. I have the Koni Reds, set on full firm. It made a huge dirrerence in how the car drives. Plus they are very reasonable cost wise.