An SBC developing a crack in the crank pulley - where it is bolted to the balancer hub - is hardly a rare occurrence. Here is a stock image:
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Cracks can also form in the grooves of the belt drive section. Here is an example (this one was found in the pulley of an engine I was rebuilding):
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Either type will cause vibration/runout of the pulley - as well as abnormal performance and reduced life of the associated belts. But, while a cracked pulley is certainly serious (and should be dealt with ASAP), it isn't nearly as critical as losing the "interference fit" of the balancer hub to the crank snout.
An interference fit means that after the two parts are assembled, they essentially become one component - with zero clearance or relative motion. In theory, the crank bolt is only required for assembly purposes and the key is only for timing mark alignment. Point is, there should never be any observable play between the balancer hub and crank snout. So it's kind of alarming when you state:
The balancer moves, too, just not as much.
Like Mason says, the sheave is only fastened to the balancer in the area of the hub:
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So, if you are able to create motion - in the outer ring of the balancer - when applying force to the pulley... it becomes a serious issue. If allowed to continue, it's possible that the crank snout could be damaged - and trust me, you don't want to deal with that.
