Here's a '69 model that you might not want to modify. Mach 1s were very popular so they're not extremely rare. However, this car has the 428 Cobra Jet engine option, making it much more valuable. Still, there are many things that you could do to a car such as this that wouldn't impact the value. For example, a larger wheel and tire assembly and a little lowering in the stance department would look good and not affect value. Parts availability for these cars gets better every day, with even a replacement body shell available. Parts availability is less than what is covered for '65-'68 Mustangs, however.