So you're saying the current trans is broke, yes?
IMO check the current gearing on your vehicle, then check the ones on the mustang, you can potentially screw yourself over if you get a trans with either too high, or too low of a first gear. No one likes to drive 3 feet and have to shift. My wife has a v6 ranger step side, and it's actually pretty decent with an auto, so the 5 speed should be even better.