Unfortunately, a lot of "facts" about muscle cars are subjective or just wrong. Was the '57 Corvette the first car to have one horsepower per cubic inch? No. That was the '56 Chrysler 300B. Both cars had it as an option. What was the first car to have it as standard, not the '62 Corvette but the '57 DeSoto Adventurer.
Was the GTO the first muscle car? Nope, that is pushed by the guy who designed it but the definition of a muscle car is a mid sized car with a big engine. The would be the '49 Oldsmobile Rocket 88. It was an Olds 77 with the Olds 98 engine in it from the factory.