Like all the other muscle cars, the closer you keep it to stock, the more its worth. Lots of guys think, cause they sunk untold thousands into their 5.0, chassis work, titanimotors, blowers, thumpity-thump stereos, whatever, the money will come back. But they are gonna be treated like hatchet jobs, no matter how well its done. That being said, do your Mustang like you want it, not like you want to make money when you sell it, and you'll be lots happier, imo. And don't expect it to be free/cheap, this is a full contact, expensive hobby.