I just used 550lb 1" lowering Mustang springs. $75 and you get improved handling as well as ride height. Also a lot less work.
I don't know what my springs were...I need to contact the guy I bought them from and see if he knows. They were MUCH stiffer, and only had 8 or so coils rather than the 12 or so the stock ones had, and were still about 4 taller than stock when weighted down. I took 1-1/4 or so links off to get the stance I have now. Anyone have any idea what kind of springs I got? The guy I bought them from said they were "351 springs"...