The 50oz balancer came out 1981 and later (5.0 engine)
The early Pre 81 has a 28oz balancer.
The counter weights are different between 28oz and the 50oz. If you have the 50oz counter weight, maybe someone put a late model crank shaft in your engine ???? Check the crank numbers to see if it is a late model crank.
Easy solution. Buy a Pro-Street or Summit Balancer for a SB Ford which has the removable counter-weights and multi-position timing marks plus they fit 3 and 4 bolt pulleys........