Stoptech did actually optimize the specific kit to work best with the Z32. They take a fully stock 300zx...do tests...size up the piston sizes by what they calculate to keep pedal feel and modulation the same as stock and then they test it. From their own website for the ST-40 & St-45 calipers: "Any combination of 8 piston sizes available to match kit to vehicle application "
Yes the ST-40 dimensions are the same for every vehicle..but piston sizes are sized according to each car....If you look at the caliper piston seals it actually has the size of the seal in mm written on it for reference.