Pulleys, gears, torque converter and shift kit will probably do more for you DOA Stang than anything else.
I think the best mod I did was the pulleys, I couldn't believe the difference they made (not collossal, but there was a noticable difference) In my experience with gears, I went from 2.73's to 3.73's and lost a WHOOPING 9
miles per hour off of my top end, and I didn't notice any real loss in fuel mileage, mostly because I didn't have to put as much foot into it during commuter mode when I pulled away from the rabble at the light.
My old exhaust consisted of 1-5/8 make shorty equal lengths, SVO off-road, and Flowmaster Force II exhaust. It helped out since my stock mufflers were rotted, but a stock powerplant isn't to taxing.
IMO unless its a custom chip, chips are for chumps, don't waste your money