Crackedback is right about measuring with weight on the axle. If everything is else is in the factory location a factory length shaft should be fine. I moved the rear end back 1/2" on my Dart and ran a factory length shaft with no problems.
As far as the rear end moving back and forth in reality it's very little if any unless the front segment wraps up. As the spring (rear segments) load and flatten out it makes the rear half longer and moves the shackle back.
