Tim, there is nothing wrong with original GM poured bearings (actually not poured but spun in).
The problem comes in when a crank in turned down. You then have to have rods re-poured. The process of pouring is not as good as Chevs spun-in process and with the journal being smaller thicker Babbitt is needed. This thicker Babbitt is much more likely to crack and shed off.
By using inserts the shell of the bearing is thicker not the Babbitt so it makes a much better bearing.
Chev never sold under size rods, you were told to replace the crank if it needed turning.

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