What year is your Dart and does it have the A body NP833 trans in it?
I have a 1969 Dart GTS original 383 4 speed car that I have two original drive shafts for, one is for the 7260 pinion yoke and the other is for the 7290 yoke.
I've had a lot of drive shafts shorten for some of my old street and strip cars, push the tranny yoke in all the way until it is bottomed out on the output shaft on the tranny,(that will probably push, crush the seal up some, no big deal on that ) then slide it back towards the rear end 3/4 of inch with suspension loaded like it would be on the road. Use the center of the U joint on the tranny yoke to the center of the pinion yoke for your drive shaft length