7 3/4 to 8 3/4 swap only

You didnt say what year anything is but you could easily run into a much larger problem than driveshaft length...I would be willing to guess that a /6 Dart is going to be SBP and its quite possible that the Duster is BBP, in other words, your wheels might not bolt back on!!! Check bolt pattern on both vehicles BEFORE you pull any parts off.

Measure from front edge of one stud, skip one and then measure to front edge of next one. That will be either 4" or 4.5"