Shimming the main caps was a common way back in that time period to adjust the bearing clearances as needed. The Chevy Stovebolt 6's of that same era came from GM that way. Maybe someone thought they were doing it like other engines were done. Sounds like it could have been a garage fix and the original crank was replaced. When the original crank was removed it was discovered to be a factory blem with undersize journals and a correct standard crank replaced it, they didn't replace the bearings so they had to shim the caps to adjust the clearances. The best way to determine your specific needs is to mike the crank journals and see exactly what has been done to your crank.
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