[Mr70]

That is early,but not impossible as these cars were built very late,and most likely the blocks were just sitting waiting for the cars awhile.
What's the Tonawanda build date before the CKO code on the Pad behind the Alternator?
Is the VIN# stamped there,or is it down by the Oil filter?
Do you see an external Oil cooler provision above the Oil filter Mount to indicate a 4-bolt block?