I think that Moog #5413 will drop the rear a bit, though I'm not sure how much.
I'm really uneasy about using coilover springs in the stock big-spring seats especially for street driving. Those small-diameter springs are not as stable as big springs.
You can get big springs in various rates at the circle track supply houses. Here's a link to one source:
http://www.resuspension.com/cart/product.php?productid=15531&cat=114&page=1
Norm