Changing out the Detroit for a Cummins can be done but it is a fairly big project. Our 664 now has a 3.9 Cummins and in the end it was well worth it. Your throttle linkages, exhaust, engine mounts, bell housing, and flywheel will have to be changed. The Detroit is solid mounted to the frame, you will need to have good rubber mounts made up to take care of the vibration of the Cummins. Also finding the right flywheel/bell housing combination can be a challenge. We may still have some of the information around someplace about what the parts needed were but it was over 15 years ago now that the Cummins was put in our 664. Even though it was a big project in the end putting in the Cummins was a good move.