This is an interesting topic. We have an 81 Imperial with the 904. It has 139K original on drivetrain. So far everything is holding up well. However given the mileage, the thought occurred to me that if the trans ever needs an overhaul I have thought about dropping in a 4-speed OD from a truck from the late 80s' early 90's before they put in the electronic controls. I cannot imagine it would be technically difficult.