The only thing I can think of is to go to a good parts store and look thru their spring books. Find our rear spring specs and then try and find something close. I did this on my dad's bonneville and found slightly stiffer VR coils that were 1/2" taller than stock. This was almost perfect as he normally has his car loaded down when traveling.
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03 GT, STB, GM HD sway bars, 35% tint, GE893(37watt)fogs, Flowtech Terminators, 180 T-stat, ZZP DP & PCM, Ceramic PLOG & Crossover, 18" glasspack, 2007 17" Mustang Bullits 235/55/17 Pirelli PZero Nero, 12" R1 D\S rotors, gutted air box, GTP radiator