Any street tire is good to start with. They don't have great grip around corners, they all understeer, and they all have relatively soft sidewalls so they will be very forgiving. Your combo is fine for autocross. A stock motor'ed GT would be perfectly fine even. The three things i suggest most in an autocross setup are tires, good brake pads like HAWK, and some nice adjustable shocks/struts so you can dial in oversteer or dial out understeer accordingly. This along with sway bar rigidity allows you to set up how your car moves around corners. Since an auto course is tight and a mustang is big and heavy you want the rear end to be able to slide a little so you can "rotate" it around turns. This requires a stiffer suspension setting in the rear that can be attained by getting an adjustable sway bar along with some good schocks.
But you don't really need to play with all that until you've gotten a few autocrosses under your belt. The main flaws with our mustangs in autocross courses are the suspension and weight. Just invest in some good brake pads and try to get all of your braking done before the turn, then feather it through the turn, and accelerate out of it. MOST of your time is based on how you come out of turns. If you come out sloppy and understeering, it will hurt time dramatically in the straight to the next turn. In other words, don't overdrive your car. Learn its capabilities. Every time i go out in mine i make a slow run the first time, and after that i make a fast one and push the car beyond its limits. In the third run i "average" the first two and find an equilibrium for that course. Just try to be smooth. People that go and burn tires and have the rear end hanging out the whole time and locking up tires have the slowest times. Good luck and let me know if you need any other suggestions.
