Wow, that seems way long. My 74 Dart Sport was only 48 3/4 inches center to center on the U joints. Did you measure with the rear suspension hanging or did you have jack stands under the rear axle tubes with the weight of the car on it?
A 74 Dart Sport is essentially a Duster and has a 108 inch wheelbase. A 65 Dart has a 111 inch wheelbase. Of course, that doesn't explain nine full inches difference.
