No, with three wires that is a Hall Effect sender. It should work as wired, since it doesn't it is likely bad. Hopefully it has the square drive piece, if the cruise works it does. Did this set up ever work?
Since the car doesn't have an ALDL connector I doubt it ever had an CM. In this case it is missing the optical pickup. There wasn't a need for it.
I'll post the speedometer and buffer box pictures in a couple of minutes. If the opening on yours is there then I'd mount the stock buffer box and go with it.
RemoveBeforeFlight