As long as the donor doors are complete it's pretty easy, you just install the doors and change the lock cylinders. The A-pillar trim and rubber has to be changed because of the wing vents. The interior panels only have to change if you want the interior to match, it would be functional without that so you could do it whenever you had time/money down the road. If the doors are just shells, you'd need to track down all new glass and hardware which would be a pain, because almost none of the internal parts swap.
I'm putting a set of '72 doors on my '74 Duster as part of my '71 Demon clone. I needed new door panels anyway, so I just ordered the correct panels for a '71. The '70-72 metal rear panels take some modification for use on a '73+ Duster, but that's because the pop-out window latch mechanisms are different. On a Dart I don't know what would have to be done to replace the plastic uppers with metal in the back.
Oh, and take a picture of your door sticker. ECS can reproduce one for you with your owner's information. That way you don't lose the registration data on the door.
