Double check the block casting number. The number given may not be a valid number. Sometimes the digits in these casting numbers are hard to distinguish. The closest number I've found is 3921968, which would be for a 64-76 230 or 250, according to Inliners. The casting date of E208 would then translate to May 20, 1968.

The given numbers by the distributor do not follow the usual engine series number format. We're these the numbers stamped into the pad just aft of the distributor? This could have been a replacement engine.

Curt
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1953 Chevy 6400, 1957 Chevy 2dr Sedan
--"Mediocrity is easy, the good things take time"