Pull pan and look at crank. Sharp edges on throws means its a cast crank needing a year correct weird balancer . If it has softer rounded edges, its a forged crank and is internally balanced. Any 273-318-340 forged crank balancer will work on any stock forged crank as they were a neutral balanced motor , everything was balanced to zero so you could mix and match.
For balancing purposes that's mostly true. However there are differences in harmonic balancers in those years. Timing marks and pulley bolt patterns should be taken in consideration too.
