Not sure about this brand of spring. I personally have tried Eibach (rank them VERY high) and BBK (ok). I hear good things about H&R. THe rest fall into i-dunno (or ok from what I hear).
I beleive you get what you pay for. and high quality springs will set level and not sag over time.
As for doing it without other stuff (shocks...) Sure you can. Just make sure that the original equipment is in good shape. Most of the aftermarket springs only increase rates by 9-10% stiffer. Should be ok.
CC plates might be _needed_ (if the alignment shop cannot get the right numbers).
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Darius Rudis
Roadracer: Home built t-arm/panhard suspension powered by an S-trimmed motor :-)
My Mustang Page