They may be 3 steps hotter than what they came with in 1968, but the fuel change required a different heat range of plug.
That 12 is exactly what he should be running. A 9 would be way too cold.
My only suggestion would be to drop the N series plug and start buying the C series plug. It has a 5/8 hex instead of the humongous 13/16 hex which is obsolete IMO.