Lincoln-Mercury stuffed a 360-hp, 480 lb-ft 429 into a 4,500-pound fastback with hidden headlights and fender skirts, then sold fewer than 8,300 of them across 1969 and 1970. A Rare Cars documentary explains why the Marauder X-100 flopped in showrooms and why it may be the most overlooked big-block bargain from the muscle era.