First I'd check out this forum for parts....
http://www.corvetteforum.com/ Look in the C-4 section and also you could put a post for parts wanted. I ordered a new rear spring from EM and it was made by Vette Brakes, I found it too soft for my liking, even with the ride height adjusted full up position. Kept hitting the inside body under hard acceleration. I had a composite spring from Vette Brakes for a C-3 sitting around, the highest spring rate they made, I believe around 550#. Put that in and solved my problem.