Only statistic that matters. Ford produced about 135,000 1973 Mustangs (last of the old body style) and about 385,000 1974 Mustang II's. Absolutely, the car was a smash hit, and the right Mustang for the time.
(Interesting tidbit: I worked at the Rouge Plant during the summer of 1974 and my job was to drive cars from one end of the plant parking lot, to another. I have literally driven hundreds of 1974 Mustang II's.).