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We'll just have to agree to disagree on that. I've balanced a lot of mopar engines and what your saying is not what i've seen in actual practice.

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Some actual numbers would bolster your argument. As you know, machine shop balance and factory assembly line balance are way different.

The forged crank for the 1972 400 has the same part number (2268114) as the 383 from 1962-1971 which tells us there is no difference in the cranks.

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John, 71 was the last year for the 383 and as you know, 72 was the first year for the 400. It stands to reason they wanted to deplete the old 383 cranks. I think we all know they'll physically interchange dimensionally. That does not mean that the component weights were the same. At the factory all they had to do was drill the counterweights differently. The pistons are not close enough in weight to just swap cranks from a 383 to a 400 and not rebalance it.
At some point you'd have to decide what is close enough, right? So in your opinion, what would be close enough? 30, 40, 60, 100 grams? typically anything more than 1 Oz.-inch will cause problems. (28 grams)

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The big issue is whether or not the crank balance is internal (no added weight to flywheel/converter) due to being forged instead of cast. Weight balance is way differemt due to offset balancers and weight pads (flywheel/converter) on the cast crank assemblies. CANNOT mix parts between the two, otherwise you will eventually have a bad vibration and a very short engine life! ( BOOM, BOOM, POW!! ) Either type of rotating assembly, I would DEFINITELY spend the extra cash re-balancing it!! Free horsepower, way smoother operation and extended engine life.