The tach is the same but the 84-86 might have a tach filter (its not part of the tach, but connected by the distributor or somewhere else along the wire to the tach).
That is what I thought, but I wanted to check. Since my dads 84 is getting a DIS engine. I changed my gauge cluster from a car with DIS when I did my swap, so it worked fine.
Actually it is getting a 2.0L OHV from a Cavalier, it looks like the same DIS setup as the duke. Engine is better than a duke, more HP, less torque, alot lighter, and stronger.