64.5 - 67 mustangs have the same problem/feature. Mustang & Fords magazine (9/02) ran an article on how to install 3 point seat belts provided by Pony Car Sales and Restoration in CA 714-639-4457. The setup bolts to the floorboard and the belt goes up over the left shoulder of the seat. The kit doesn't look Mustang specific but I've no idea what the seatbelts in a Dart look like. If you want a scan of the article let me know.