Here's the problem:
A Fox body is an old car. A crude, old car with a cheap V8. Granted, it's not an STI, so you're probably not going to wrap it around a tree, but this is an investment, right? You want to build it into something nice, right? So take up Granpappy's budget offer, get that Mustang, garage it and work on it, and buy something horrible... like a Chevy Prizm (Corolla dependability at rock-bottom depreciated prices) to putter around in while you earn your license.
This means that all the inevitable dings, dents and scrapes you get while learning to drive go on the grocery-getter while the Fox body gets to come out on those twenty Sundays a year when you can actually enjoy it without fear of the other idiots on the road.
That last part is important. When we say underage drivers are at risk, it's not just because you're still learning how to stir the gears and keep the car in a straight line. It's because you still don't have the experience and situational awareness to avoid inattentive and dangerous drivers. No matter how good you are at the actual job of piloting, situational awareness takes a few years to develop.
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But go ahead and do whatever you want. Just make sure that woman who T-bones your precious Fox outside the school parking lot has insurance.