There may be a number of reasons for this, but first I would say that you take notice of the difference and don't use the wrong pulley/harmonic balancer.
One reason might be because they require a different balancer because one engine is internally balanced and another is externally balanced, like the difference between 318s, 273s, 340s and 360s.
Another reason might be because of different harmonics in cast and forged cranks. Or it might simply be that the different capacity of the engine requires a different harmonic response.
If you put in a call to Chris at BHJ Dynamics (look them up online) he would probably have the answer at hand for you. They make both OEM-style balancers and race balancers for most of these engines.
Which brings to mind the difference between two of their balancers, their 318 balancers. One is made with the timing marks in one location and the other is different because Chrysler changed where they put the marker on the timing cover.
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