The third-generation Impala arrived in 1961 looking like nothing Chevrolet had built before, and it produced two body styles that collectors now hunt specifically because almost nobody wanted them when they were new. It also produced a 409 that got its own hit song, a dashboard where the temperature gauge was replaced by a warning light, and a 1964 engine option good for 425 horsepower. Read on for what separates the good years from the great ones.