Greg, since the EUN is on the PHS, that is compelling.
If the block is original to the car, we already knew the VIN would be an anomaly.
John might be able to help figure out if the head dates are reasonable for the EUN using his EUN database. I don't know if he has good data for the Production Dates that builds were getting a particular EUN, but if so, that is the data that will help confirm "good" dates for the heads.
Since John is saying 10B as the earliest Time Built code for your EUN, I'm thinking Oct. 6th and 7th for the heads might be right in line with an engine assembled around Oct 10 and ready for installation in a 10B build so the head dates could be right. Except the engine assembly didn't go into a 10B build and for inexplicable reasons hung around in inventory until winding up in your May build.
A possible explanation, your engine was assembled and ran so poorly on natural gas in the engine plant, that your engine, originally destined for a 10B GTO, never got there and was plucked off the engine assembly line and set aside. Finally somebody fixed whatever was broken and put it back in inventory, whereupon it was placed back on the line and installed in your GTO in May. Whatever the case, it seems to be just an anomaly.
Greg, what about the cast date on the intake? I have seen lots of guys misread cast date tags on blocks, 5s look like 6s and so on. I believe the '68 cast iron intake manifold is the same 9794234 as used in '69? If you have misread the cast date on the intake, could be the intake was the problem that was fixed before it ever went into your GTO and would have a May date? Worth another look.
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