Drivetrain swap/ wheelbase

The trans will bolt right in. The differential will bolt in but you need to swap the shock plates as the 8-3/4 has 3" tubes where-as the 7-1/4 has 2-3/4" tubes. The 68 Cuda is a 108" wheelbase car and the Dart is 111" wheelbase. The 904 trans. requires a longer driveshaft (not exactly sure but it's somewhere between 3-1/2 and 4") and the 8-3/4 requires a shorter driveshaft (I don't remember exactly but it's around 1-1/2" difference) than a 7-1/4" equipped car. Unless you get extremely lucky the driveshaft probably won't be a bolt in. Sorry I can't be more precise on the measurements.

Check out this thread and you might be able to figure out what length driveshaft you need so you can compare it to what your getting

http://www.forabodiesonly.com/mopar/showthread.php?t=139090