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Roy, shocks and springs are designed to work together in a set. You are asking for a softer shock. Well you can buy a set, but the design of the shock is to control the spring, and the softer shock won't do that well. I don't really suggest that you change the suspension, because that will lower the designed-in performance level of the Z06, but if you still insist, then go to Vette Brakes or other manufacturer and get springs and shocks designed to work together.
Roy,
I tried Bilstein HDs on my Z and they gave a superb highway ride along with maintaining control of the car. I think the Bilstein HDs will give a Z a sport touring ride as opposed to a track ride. I ultimately went with 2004 ZO6 shocks for handling performance reasons. You will not be dissapointed with the Bilstein HDs, they are softer than the Bilstein Sports by about 15% and Bilstein Sports are similar in ride stiffness to the ZO6 shocks. Good Bilstein prices can be had at http://www.eshocks.com/