Any definition of "mucsle car" written after 1964 is irrelevant. They are strictly opinions, having no basis in fact. The class was firmly established by GTO.
'71-'73 Mustangs are nowhere near the size of intermediate cars. The wheelbase is still under 110", and the weight is under 3,500 lbs. (moderately equipped). GTOs and Chevelles have a 112" wheelbase ('68-'72) and weigh well over 3,600. Some of the 429 cars were pretty fast, even with the lower compression, but the small block (Cleveland) cars are no match for a muscle car. The "flat back" cars are my personal favorite of the Mustangs.
We can banter about this all day. It doens't matter. My purpose is to dispel myths and superstitions (why I post on these boards). My experiences over the years add up. My brother is a Mustang fanatic, but even he conceeds, they're not muscle cars. He's seen the tail lights of GTOs and GTXs too many times. He has quite a "stable" now, too. A '65 "K-Code", a '78 King Cobra he bought new (an award winner at the '04 Nashville Birthday Party with one of our engines in it), a '99 Cobra (will STOMP the '65), a couple of Fox-bodies and a REAL Bullit (not the "new Bullit").
I like Mustangs just fine. But the name of the class is "Pony Car". That name is for the premiere car in the class, Mustang. Had the class been named for the FIRST sport compact-based "performance" car, it would be the "Fishy Car" (Barracuda was released before Mustang). There were certainly Firebirds and Camaros with more muscle than ANY Ford of the era, but they're not muscle cars, either. They're "Pony Cars", too. The last MONSTER car of the era was a Trans Am ('73 and '74 "455 SD" cars). A Pony Car... GTO never got the "good" engine after '72.
Jim