Paul and Dave,
Read the post above . . . what "Coosawjack" is saying could quite possibly be the answer. Early blocks were known for variations in casting and machining processes, therefor, rope seals are the only way to go cuz the neoprene ones depend on the block being precise . . . Rope will conform to just about any irregularity and once it swells from the
oil, will seal perfectly. I've built both normal and reverse rotation engines and always went with the rope for early blocks. They do cause more drag on the crank but with a 427 as the source of power, who cares? :>)
Doc