I was just at the UPR products site before this and they have a coil over kit. Actualy there are quite a few. If you don't get 5.0l Mustang or Muscle Mustangs magazines go to the store and pick one up. Or scan though it. There should be adds throughout that have them... I don't know all that much about the difference between brands or all that much about them though.
They sound pretty neat. And at least on some if not all you can adjust ride hight.
I have just ordered UPR springs for my 89' 2.3l coupe... $150.00!

Lowers the car 1.3"-1.6" Pretty good price. They are made by Eibach and I hope that they are basicly like Eibach ProKit Springs which can usualy be found online for $220-240. We'll see what I get. I plan on ordering some KYB gas shocks also.
Should make my coupe sit just right 17" X 9" 5 lug Polished cobra R's I found for $150. (I had to file and sand the lips because of some curbing and Polish them a ton)