Tough to say, because in addition to the smog equipment, they also changed the horsepower ratings from SAE gross to SAE net between those years which was the reason why mid 70s horsepower ratings fell.
If they were perfectly restored, the Mustang would win. By 1975 the big blocks were hopelessly choked with smog equipment, while in 1972 the emissions equipment was pretty simple.
Trans Am 16.9
Car and Driver best times, Mustang 351 Boss engine.