Chevrolet’s factory SS 427 package turned a handful of full-size Impalas into genuine muscle cars between 1967 and 1969, with roughly 6,350 built across all three years combined (1,778 of those in 1968 alone), rated at either 385 or 425 horsepower. The sleeper pictured here takes that idea further, pairing period-correct Impala sheet metal with a fuel-injected 496-cubic-inch big-block and a modern 4L80E automatic overdrive transmission. The result looks like a family car and drives like anything but, continuing a sleeper reputation the factory only hinted at fifty years ago.