Roll down the windows with the crank handles in the doors, if the air coming in isn't cool enough, push down harder on the loud pedal.You may need a long stretch of road to get the air coming in cool enough...but thats the fun part!
Are you sure it even has AC? Big box under the dash with vents ! Red is hot blue is cold , fan speeds are self explanatory, there was no ' climate control ' in those days . Pick a setting and pick a fan speed . Otherwise you have a simple Mustang with heater contols , same thing red = hot blue=cold and no AC