I have a 65 Dart and a 64 Valiant. The 64 Valance is much different, with a single wide opening going almost to the edge. The turn signal is fairly square, with a curved outer side that fits the valance. It looks almost new, but probably original since the connector is correct. It sits about where the round hole in your Dart's valance is. Unless the 64 Dart is different than a Valiant, I would say "won't fit".