Your right it is a straight 6 and a /6 fits but there's a huge diff. in the bore diameter of them so the Cummins is a bunch longer. I'm gonna take a guess and say it's at least 6-8 inches longer. Since the V10 barely fits due to length I can't imagine even being able to squeeze a Cummins in there unless the radiator yoke and firewall was severely modified. Your right it would be cool though.
Cool pics of the V10 Dart. That proves it can be done.