OK, lot of strange information on this thread. Some of it not completely correct. I'll start with the harmonic damper, usually (incorrectly) called the harmonic balancer.
The damper is there to catch a resonate torsional vibration and keep the crankshaft from breaking. If you take a long socket extension and try working a stuck bolt you will notic the shaft twists and will spring back. Now find the frequency that the shaft will resonate at and add a little more energy at the peak. The cumulative addition of energy will build up and break that shaft. The crankshaft is that socket extension and the firing pulses are the source of energy. The damper works by the inner and outer rings on the rubber center cushioning that torsional vibration. It is covered nicely in a book called "Mechanical Vibrations" ISBN 0-486-64785-4
Same book will also cover the balance issue. This is usually easier to understand if you think of a tire that is out of balance. There is a heavy spot that tries to pull the tire off the center axis. To balance a tire you counter that heavy spot with another heavy spot (wheel weight). The early engines have a rating of 28 oz-in and the new engines are 50 oz-in. That oz.in is ounce inch. one oz-in is 1 ounce of weight 1" from the center axis of the spinning shaft. In reference a moderate sized tire weight is one ounce. on a 15" wheel that is 7½" radius. or 7½ oz-in imbalance. The difference between the old and new is 22 oz-in, about 3 times the imbalance of a moderatly out of balance tire. Now keep in mind a tire spins fairly slow, a big truck tire could reach 1,000 RPM at around 100 MPH (depends on tire size). Really it sees a lot more around 500 RPM. Engines turn 5,000 RPM. So 10 times the speed. Or another way of looking at it is what a tire spns going down the highway is about what an engine does at idle, and the forces go up as speeds increase.
Also, there are plenty of 70's cars and trucks with 4 bolt early balance harminic dampers. Also all 351 engines use the early 28 oz-in imbalance, and those stayed that way until the 90's. So being 4 bolts does not indicate what balance the damper is. It can go either way. But the 3 bolt was discontinued before the 50 oz-in was made. So there are no factory 3 bolt 50 oz-in balancer to shop for. A good machine shop can change the balance of a damper from one to the other or there are aftermarket dampers to take care of this swap issue.
So simply, no cou can't just bolt on the old 3 bolt bronco damper onto the '84 302 engine. There will be problems.
