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I also agree that .100 walls are ok and you will safe at .120.
Although sonic testing was not available in the 50's & '60's I think a lot of guy's got away with ~.090. I know the most common over bore was 3.375 back then. I had a '48 Merc bored 3/16 and most of the hot FH's were 3/8 x 3/8 (3.375 x 4.125).
As a hot rodder and a mechanic, in those days, I don't remember split cylinder walls.