If you are looking for pure financial gain, it is a terrible idea. I don't see any way that you could make money on a run of the mill GT. Say you spend $30k on a new one and pay cash. After 20 years, that $30k would've been worth $40,400 if you left it in a savings account with 1.5% interest. Keep in mind, that is a very low interest rate and it would likely be higher average over that long of a time period. Now add in the cost to insure the car for 20 years. Even with bare minimum coverages, you'd still probably spend maybe $500/year average to insure. That's another $10k, and again factor interest you COULD'VE earned on the money you are paying along the way, and that makes it more like a cost of $12k. Let's just forget maintenance costs and the cost of the space you are using to store the car.
So far we are at around $52k to break even, without maintenance costs. If you have to finance the vehicle, that makes it even worse. That is for a 20 year time period. 30 years adds another 15-20k to that.
Now let's look at what run-of-the-mill GT's are worth that are 20-30 years old. This is a bit harder, but ebay and whatnot can help...
1991 Mustang GT Convertible, 25k miles: $12,888
1988 Mustang GT, 1500 miles: $10,500
1988 Mustang GT Convertible, 10k miles: $17,500
1979 Mustang Indy Pace Car, restored: $7k
1978 Mustang King Cobra, 130k miles: $7500
Even a 1993 Cobra w/ 45 miles is going for $24,500 (buy it now) - that vehicle's MSRP was right around $20k and only 5,000 were made in total.
Those mustangs values are low because they aren't collectible or desirable. They are just plain-jane old mustangs. In 20-30 years, so will a 2010. It has no special or unusual features, it isn't rare, and it isn't unusual. I don't even really see something like a Bullitt being worth much more, since really there is little difference between it and the stock GT. And I DEFINITELY don't see one of them being worth $50k+ to make it a worthwhile investment.