Thinking about it, The '64 GTO comes across as the first planed muscle car. Then as things got moving, bigger cars with bigger engines were pretty muscular ( 442, the Fords with their 427, Chevy's 427 cars and more). Now the thinking part, if these cars were muscle cars why not the older cars with big powerful engines (409, Ford's 352,390,401,HEMI letter cars, rocket Olds)
Now were talking muscle here, why not smaller engines? The 289 Cobra and a bunch of Corvettes were quite muscular.
But by that , does a 318 Charger fit in ? How about a 1969 383 2bbl single exhaust Charger ?
Where do the Trans AM, 302, Boss,Z-28 small blocks fit ? They are muscular.
Does it mater what vehicle this engine is in, at one HP / one Cubic Inch ?