I kinda went bonkers with my
suspension set up. I have A lot of experience with 99-04 style mustang suspensions! you will never get the ride height you want just by buying springs. There are other parts you need to make them all work in harmony. The springs you got are probably set for a ride height that will not mess up your alignment and handling too much. When you get aggressive with ride height you have to start to work on things like bump steer, caster/camber settings and how your car will behave when really low.
The first mod I got was this....
http://www.maximummotorsports.com/1994-98-Mustang-1999-04-GT-Starter-Box-P185.aspx This kit will give you a nice drop but nothing to aggressive. you will probably still have about 2" wheel gap in your wheel well. This kit will use the H&R springs. I use the isolators too. If you are to order this kit, explain you want to upgrade the fronts to coil overs and they will adjust the price accordingly and add about 200 bucks to the order. They are really cool with answering questions and see if the spring you currently have will be ok for the rears. if so ask to have them removed from the quote too. then you will save a hundred bucks or so. The other thing you need is the lower rear adjustable
control arms they are like 3 or $400. you can get them at maximum motorsports too. then you can adjust the rear height and front to whatever you want. You are looking at about $2000 in parts and about 2 hours tops per wheel for labor if you have a slow mechanic.
The above option is not perfect for an aggressive drop but at least with the kit you can adjust your caster/camber and that will make your car very happy.
Another thing to think about is that the lower you go the more stiff the car is going to be, if you do several hours of driving everyday in the car, the last thing you want is to be uncomfortable.
Wheel gap will look like this eventually
The front wheels are high enough that I can get over speed bumps and barely touch the cement parking blocks at the local parking lots. Thats why the front looks to be a little higher than the rears.