H&R Super Sport Springs (94-04 Mustangs)Front: 700-760 (progressive)Rear: 275-300 (progressive, solid axle)
Mustang GT/V6 Vert. 1994-2004400 (linear)185 (linear, solid axle)
There's a comparison of the spring rates (in pounds/inch).
Springs like the Eibach pro-kit don't raise the spring rates much, so the stock dampers (shocks/struts) can do an OK job of damping the spring oscillations, but when you bump up the spring rate by as much as 90%, the stock shocks and struts just aren't going to be able to effectively damp the springs.