'84 BII should be a 2.8L.
But yeah, a 2.9L or 4.0L M5OD should fit the 2.8. I'm not real sure, but there may be a difference in the clutch slave setups though. If you have an external slave, you might need to convert to the later concentric slave & TO bearing, but I don't know 1st-hand (maybe someone else can chime in on that).