Unless this car has EXTREME sentimental value to you, it's probably not worth fixing.
Like Grumpy said, it can be done. With enough time and money, anything can be done.
A 88 Mustang isn't a rare car and you would spend a ton of time and a bunch of money to restore a car that you could easily replace with a nice example that doesn't have a "welded on" roof.
Unless this car happens to be a Notchback SSP, you're just throwing good money after bad, IMO.
The barn on the other hand... Now that is worth rebuilding!