First question is--are you sure that the springs actually came from an Impala SS, and they were the original springs--was there still a factory ID tag on the spring(s), and what does it say?
Second would be an observation--the springs may not be properly indexed back into the pocket on the frame or in the lower control arm--there are 2 holes in the arm where the spring rides, and they are used to check the position of the spring--one hole should be covered by the spring and the other should be clear--it also acts as a drain hole for the spring pocket.
What shocks did you use? I am not necessarily thinking that the new shocks may be "holding up" the front end, but it may take a little time to let the front end settle, between the spring and the shock. Your description sounds like some other problem, however.
Let us know what you find out!