Safe to bore a 383 60 over?

The "Thin wall" claim has been in many published releases from Ma Mopar. I don't know if it was printed in any factory service manual but in Mopar approved rebuild manuals, it has been printed there.
Mopar Muscle and Mopar Action magazine writers both have done stories where they sonic checked cylinder wall thicknesses on a variety of big blocks.
Both of them arrived at the same conclusion. Mopar Action's article elaborated a bit, stating that the false "thin wall" claim may have been a disguised warning due to a slightly inferior metal used in later blocks, a lower nickel content in the casting.