How interchangeable are 318 harmonic balancers?

318s are all internally balanced, so the harmonic balancers are interchangeable, with 2 caveats:
The bolt pattern- '69 and earlier had one offset bolt, '70 and later were perfectly symmetrical.
The timing mark changed location in '70 when they changed from the cast iron water pump with a drivr's side inlet to the aluminum pump with a pass. side inlet.