Sounds like you covered the basics. While it's certainly possible that both your original and the replacement cluster were bad, I don't think it likely unless it is a common problem to that model. There is always a possibility that there is a difference in signal between year models. Otherwise it would appear that whatever triggers tachometer from either the injection pump or elsewhere is fried.
"Cheese Wagon" <firstname.lastname@example.org>
Former owner - 1989 Ford B700 64-pass Blue Bird (Rest In Peace, Cheese Wagon)