Here is a nice chart. All the spring rates seem to be pretty close.
http://atlanticz.ca/zclub/techtips/springs/index.html
In talking with Sal at MSA the Eibach fronts are linear & the rears are progressive. This chart bears this out. I ordered the Eibach's, Tokico HP's and bump stops for the '78 280Z today. Hopefully a 1" drop will be just about right for the 280Z. FWIW I'll be selling the KYB's I am currently running. They are basically brand new with very few miles on them. PM or email if interested.