Contact these guys. They have all the original specs from the manufacturers and build them for you, to order. I have had them make several sets over the years for my various Firebirds and they can also make them in any modified version you like. I have usually asked for the stock spec and then told them to make them 7/8" lower ride height and they were dead on perfect.
https://www.coilsprings.com/classic.aspx
here's a link to one of the sets I had them make:
https://www.yenko.net/forum/showthre...ng#post1341916