Other then the brand of pads to use there's not that much you can do to an IT brake system that is in good condition.
On a Mustang, and I have owned 3 of them, you can make some minor improvements.
Remove the splash shields on the front brakes. That's a simple one and should be done to any car that is raced.
Add brake cooling ducts to point at the back of the rotors.
The front calipers are located on sliders that have rubber bushings. Replacing these bushings with ones made out of stainless steel or bronze will reduce the taper wear that occurs to the front brake pads. These metal bushings used to be available from most any FOX Mustang parts vendor. I can't say if that is true today.
Racing brake pads to fit the calipers that the '79-'86 Mustang GT and all FOX 4 & 6 cylinder models had are not common. You may end up having to have some pads custom made. But again I'm not up on what's available currently.
Other then replacing the flex hoses with braided SS hose there's no special secret mods that will make a Mustang brake like a sports car.
Since you can add suspension links to a car in IT but can't remove any you should add a panhard bar and consider going to a PM3L. This last item stands for "Poor Man's 3 Link" This is done when adding a panhard bar. You replace the bushings of one of the upper links with foam rubber and the other with spherical bearings or only one end and leave the other the stock rubber.
Visit Corner Carvers.com and do a search on the forum using the search work "PM3L". You should get plenty of hits and plenty to read.
