The wheels won"t fit the front with out a spacer they will hit your brakes.Newer mustangs use a different back spacing then the older ones thats why you need the spacers make up for the differenece.

 

Late model wheels will fit on the car with spacers. Please note however you will need to purchase new tires since the tires on the late model wheels are much larger than the 26 inch OD tires the classics are designed for. This in my humble opinion is the trap/fallacy of the late model wheels. They are often considered as a 'cheaper' alternative because "look its like $500-600 for these take of wheels and they come with tires!".

Then you spend $150-200 on adaptors to make them fit the car
Then you end up buying new tires anyway

Once you get to that point you have already made the capital investment so you use the wheels. And there is nothing wrong with using the wheels they look fine on the old car, but just make sure they are the wheels you really want and not the wheels you think look ok and are cheap to buy.