I know for the higher end zeals like the Function R they are all special order, you get to pick the spring rate and they custom valve the shocks to meet the ordered spring rates. If you wanna do something really cool do something like what ZZYZX does for Evo's and Impreza's that allow you to use the Koni 8611 double adjustables.
SP-Tec also makes front strut casings that allow you to use either the Nismo N1 inserts or AE86 HTS inserts. Dunno what you would do for the rear though.
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