Jeff,
The organization of a typical Shop Manual comes to mind.
First, volumes are broken down by year and model. Then there are about 22 divisions by components, such as:brakes, transmission, cooling system, body, etc.
On top of that you also note the plant and build date.
That being said, there's probably enough similarities in 1964.5 - 1966 production to group it together. If you post an article about something pertaining to a 1965, it might apply "somewhat" to 1966 as well. Then there are instances where there are distinct differences between 1965 and 1966.
There may be no "best" solution to this. But since the purpose is to find and readily retrieve information, a grouping similar to the main forum index may be in order. People are already familiar with that structure.
If you choose, you might make it a hierarchical listing and have files nested within files - so searchers can drill down right to the information they're looking for.