Headers and low restriction mufflers will help a lot,a high performance cam even more.. another way to increase performance is install bigger (lower) gears in the rear end.. 3.50:1 for dual purpose and 4.11:1 or higher for drag racing..it comes stock with 3.00 gears..
Think of your 'Stang like you would your body . . . you are replacing your carb with a potentially higher performance unit. Will this really improve your performance? Probably not, not if that is all you are doing to it. It would be like you going to the gym and using only the treadmill at it's lowest elevation. But that is all your stock body can handle. Over time you will be able to increase the difficulty level a little at a time, but progress will be quicker if you start lifting some weights and add other routines into your workout.
So how do you improve your car's performance? Like this: you will have to improve the engine's ability to breathe, and that means beefing up the valvetrain, changing the camshaft and lifters, replacing the weak, tired valve springs, and perhaps opening up the exhaust system by using a less restricted stock muffler and perhaps adding exhaust headers to scavenge the spent gases more quickly.
Unfortunately this is how it works in the real world, there is no one-step quick fix, only by carefully balancing all of the systems involved, intake, compression, spark timing, and exhaust, you can achieve the results you are seeking.