I go to Bagge and Son on Washington and Overland in Culver City for all my spring-swapping needs (they've done it for me three times now). They're good guys and straight shooters. Besides my car, I know they sometimes work on someone else's Impala for Autocrossing.
Their number is 310-838-8485. Mention my slammed white caprice and they'll probably know who sent you.
Dont drop it too low... I think our cars look great low, but Ive got the H&R springs and I scrape tire on wheelwell turning on some of our lovely LA roads and high-centered in a Manhattan Beach driveway once. Were I to do it again, I wouldnt go any lower then Hotchkis springs in this town, if even that low.