Well since JD Adams hasn't been around in awhile I'll take up for him.
Your best choice is the Bullitt suspension kit. It comes with a matched set of springs, struts/shocks, and sway bars. I got mine delivered to my door for $416 from Summit. Gene Evans Ford has the kit for a little less, but once I added their shipping Summit was cheaper for me. Just changing springs without doing the rest of the package can/will result in a harsh ride. I had the Motorsport springs in my 89 with stock struts/shocks and it was a terrible ride and the handling wasn't much improved either. I just put the Bullitt kit on this spring (after reading JD's praises) and its great. It handles much better than stock, and the ride is firm without being harsh. With the road conditions here in Michigan that's a darn good thing. If you don't have a 17" wheel and tire package you might want to consider that upgrade as well. The Bullitt kit drops an SN95 3/4-1" . Dropping a car much more than that brings a lot of suspension alignment issues that can require added parts like camber/caster plates and different ball joints to get the geomerty back in line.
MadDAWG