How did you determine the Moog spring rates were correct? I'm going through the same issue with the z95 springs. Discontinued, but I don't want to order the wrong aftermarket ones either.Those OEM Z55 coil springs are no longer manufactured by GM (Why? Who the heck knows!). I just went through this, and I had a dealer check the dealer inventory nationwide. There were 8 OEM coil springs in dealer inventories nationwide, and only 1 dealer actually had 2 in stock. The dealer price plus the added freight to get them to me, made them a non-starter. I purchased the Moog coil springs at Rock Auto, and so far, they work fine.