Jeff,
I am sorry my phrasing of the question may have upset you. I merely was just using the 66 GT350 as an example to use in a question for clarity. With the additional information I later found in Brant's article I offered it in conjunction with your response.
Long day and your request was specific so I researched and responded accordingly "If a GT350 had a scheduled build date of 1 Oct 65, what appx date code would the original 289 engine be?...."
Brant's information (given your question) did not relate to your question as you posted it so it appears to be unrelated and a distraction from your stated focus. We can only read and respond to what you post
As mentioned you will find tons of responses of "the dates
could be as much as 90 days prior" if asking about all K code production for 64-67 production. Not sure why we would limit a discussion to only 65-66 since for 67 we have much more solid data since we know when the cars were built.
Nothing I've seen nor have workers at the engine plants stated that I'm aware of suggest that the engines were built in batches and stored at the engine plants and the car plants didn't have extra storage areas. Its most likely IMHO that K codes were built at the same time as A and C codes just at a lesser rate. If you notice in the picture of the loading dock at the engine plant from 65 posted in that section of the site you can get an idea of the ratio of all the engine and transmission combination was on that day as they were preparing to load a bunch to be sent to one of the car assembly plants.
"Did Ford produce, date stamp and assemble HiPo engines in every month of 1965 and 1966?" Looking just through my records for 66 I found engines cast during every month except I don't have examples from June or July but expect that they are out there. You might want to run a survey over on the K code site and through all the threads as well as the for sale ads to collect and compare dates
If we instead (sorry for getting in the weeds but this is all about the details) "how long before the car was finished
were the engines cast or built" that would likely lead to focusing on a specific time period (year and month) and examples built during that time period discussed. Data appears to suggest that weather, time of year, rail company issues, Ford workers union issues and many other things could affect the span