If the Mustang is structurally sound, it could bring $10k to $20k depending on condition. Prices jumped a ton many many years ago, about the time the TV auctions became popular. That's also when it no longer mattered if a car was altered, big prices since then for any modifications.
Restored means nothing now, the definition changed, so ignore the abused word restored. It used to mean no non stock parts, OEM tires and batteries, no new paint etc. I personally don't mind modifications, but I don't appreciate destroying the meaning of words. I know what an air cleaner is for example, I doubt anyone under the age of 30 really does(same for the word intake).