I'm a numbers guy, not because I place great value in them but because I am fascinated by them.
I don't have any special insight for '65 but I have an extensive log of '64 engines by Engine Code, block cast date, Engine Unit No., and where the engine could be determined to be original, the Time Built code, Car Model, and Final Assembly Plant.
As my log shows, most '64 blocks moved pretty quickly from foundry casting to engine plant assembly, to final plant, and then installation into a particular build.
It is pretty easy for me to make a pretty close estimate of a block cast date from the '64 EUN in most cases. I have uncovered anomalies but they are exceedingly rare.
I can also generally ferret out obvious restamps with the data I have.
As to the "lead" time between the block cast date and installation in a build, the norm is pretty short.
But again there are exceptions.
The longest lead time that I have logged for '64 was for a 4 bbl 326 HO optioned Lemans built at Fremont.
For '64, the Fremont PHS records identify the original EUN.
This particular engine was assembled in March as made obvious by the EUN, the block cast date was mid March but the car it was installed in was built at Fremont in very late May, approximately 9-10 weeks after the engine was assembled and 10 weeks after the block was cast.
The 326 HO was a far more rare option than the GTO option so that would be one explanation for why this particular engine languished in inventory for as long as it did. However, not all 326 HO engines exhibit that kind of spread so more likely it was just an anomaly.
Without the Warranty Booklet or some other documentation to show the original EUN, nobody can really know if an engine is correct based on cast date.
I usually advise that as long as the Engine Assembly existed at the time the car went to final assembly, anything is possible.
Some are more plausible than others, but that is a minor quibble in my book.
I have not logged any '64 GTOs with such a spread but that doesn't mean it didn't happen.
I used to believe that the satellite plants such as Fremont would typically show wider spreads than the home Pontiac Plant. However, it turns out that even this isn't necessarily so.
A 2 month spread for your May built GTO would be unusual (at least in '64 but probably also true if you logged '65 data). But I wouldn't say it wasn't possible.
Just my opinion!
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