Personally, I would get 450-580(in that range) springs. Stock is 430-550, so thats a 10% stiffer spring. GREAT choice for a street car. In fact, I had a set of Eibachs in that spring range. THen moved to 650# linears for more racing, now moving to 880# (400# coilovers).
Get some from Eibach, H&R (not recommedning the "B" springs).
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Darius Rudis
Roadracer: Home built t-arm/panhard suspension powered by an S-trimmed motor :-)
My Mustang Page