Any decent alignment shop CAN align a Mustang but they may not WANT to do it. The details of how to do it are pretty simple but, compared to many new cars, all those fiddly shims are too much trouble. And after 45 years they also may not want to deal with a car possibly having been hit 3 times from 4 different directions which can make a proper alignment more difficult, it not impossible. Impossible to them usually means it would be so much work that they would lose money doing it so 'please go away'.
As with any car the alignment specs depend greatly upon how you want the car to handle. The ones Paul gives are close to what I would also use but they are not the Ford specs. From the manual, the 'desired' caster would be Zero degrees and camber would be positive 0.5 degrees for a V8 car. So when you say 'specs' you need to define what you mean by specs.