mid 70's 360 harmonic balancers

So the harmonic balancers are not interchangable thoughout the small blocks? I am aware they use weights on the torque converters as opposed to the holes drilled in the flywheel. I guess what I am getting at is, Is the harmonic "balanced" to the 360 and only usable on a 360 or could a person use the 360's harmonic on a 318 or a 318's harmonic on a 360? And are the harmonic balancers from the magnum engines different from the LA engines alltogether? My concern is due to the possibillity of picking up an sfi balancer (used) for my mid 70's 360 and I want to be sure I am getting one that is compatible to my engine.