No matter what your plans for your Mustang, there are a few items everyone can agree on. A set of floor mats? Makes it easier to keep your interior clean while adding a touch of personality. A nicer shifter? Considering how often you shift per a drive, you'd be crazy not to upgrade it. But from there, interior improvements start to become more about you and what you need your Mustang to do.
If you're dying to hit the track, then you have a list of interior upgrades you'll need to make. Racing seats, harnesses, and roll bars are critical safety upgrades, that just happen to look awesome when they're installed well. These few items immediately show that you're serious about performance driving, and you got a Mustang to go fast.
If you're interested in speed but aren't interested in organized events, then you're more free to select just the items you care about. Race seats are available in a variety of styles, and without rules, you can choose the ones that look best to you. Gauges let you keep track of your own performance, so you can compete against your own metrics and collect the data you care about.
Your Mustang is ready to become your dream car, you just need to pick out the parts that will make it fit you.