I don't think there are any after market pans that are right for an early Dart. All the pieces I looked at were for a later A-Body or for a Valiant/Barracuda. The Dart floor is longer than a Valiant/Barracuda due to the longer wheelbase. I don't think anybody makes the extension for the rear foot well of an early Dart that adds the extra length.
Not that you can't make them fit, but I went with an original floor from a donor car.
http://www.forabodiesonly.com/mopar/showthread.php?t=196103