Ask anyone to name an American muscle car and you will hear the same handful: GTO, Chevelle SS, Road Runner, Mustang, Camaro, Charger. They earned their reputations. But the late 1960s and early 1970s produced dozens of other cars with the same big engines and, in some cases, better numbers, and most of them never got the same fame. “Underrated” is a judgment, so here is how this list was built. Every car on it…