You want as much travel as you can get, and it is usually limited by the shock travel. On an early Mustang you can get some additional suspension travel by notching the frame rail where the upper control arm hits it at full extension. If you take the shocks off and jack up the front end so that the control arms are at their limit you will see where you can cut a couple of notches in the lip on the frame rail that will allow for further drop. Once that is done you want to take full advantage of the travel by fully extending the shock and modifying the upper mount so that the shock is at full extension just as the upper control arm hits the frame rail. I don't like having the suspension hang from the shock. Once that is accomplished you want to lower the front end (trimming coils from the springs) until you only have about an inch of down-travel on the shock when the car is static and level. Trim the upper control arm rubber bumpers accordingly. That will maximize the amount of front end travel on launch.
You want the shock to be really loose on extension. The Calvert shocks are cheap and work like a charm. If you want adjustables I have some Koni SP1A units that I will let go at a reasonable price.
The other thing you should do is minimize resistance in the control arm and spring perch pivots. Get roller-bearing spring perches and low friction control arm bushings. OpenTracker makes some nice stuff for the Mustangs. Get in touch with Alex Denysenko at
www.moneymakerracing.com and he will have you dragging your bumper in no time (with an empty wallet and a couple of busted knuckles).