well... if you want it to sound more like.. say a 68... you can do a lot.. of course different exhaust.. but really... you need to do sometimes to the engines internals to give it a deep rumble... such as a hotter cam, some after market headers, shave the block to give it higher compression... of course remember.. anything done to the engine will also give it a much lower reliability along with worse fuel fuel mileage and the motor will not last as long... exhaust will probably be the easiest to do and will have no effect on your motors life span BUT will raise your fuel mileage and horsepower... you can also add glass packs to.. which will give it that crackling sort of sound you hear most often on the tuned up V8 Camero (well i have seen a lot with that option at least around where i live) i gave you just one site i found them for sale on.. just search glasspack muffler for 08 mustang.. you will get a lot.. i hope this helps!!
A: get 3" exhaust with glass pack's or turbo mufflers. If your not so much in the way of having a bunch of cash. Go down to the Auto Parts store and buy yourself a Resonator to attach to the end of your tail pipe.