4. And the II designation IMO. It's easy to say that it's "not a Mustang" with the II after it. Ford kind of made it the redheaded step-child when they did that.
Besides, the performance image steadily grew as the years went on. The 64-66 coupes were called secretary's cars - how's that for a performance slogan? Sure, they had a performance version in GT form, but not all Mustangs were designed to be race cars, they were made to be driven from point A to point B and look good doing it. Besides, the 70s were a dark time for ALL automakers as far as their performance image goes. Most everything was choked because they were trying to figure out emissions and how to achieve better mileage. Back then, performance meant sucking fuel through a fire hose, that doesn't bode well for trying to maintain fuel economy. We live in some good times now and look back at those days through our modern eyes. "It only had 139 horsepower" because of a multitude of things - but, it is still a <3000 pound car with a V8 - power to weight ratio is where it's at! The only Mustang to weigh less is the new-for-79 Fox body, and it wasn't lighter by a whole lot. Transplant a later model 5.0 from a Fox and do the same mods you would if the engine were still in the Fox. There would be nothing wrong with the redheaded step-child then - except the II designation....