^^^ Exactly. People forget that assembly plants were just that, assembling the cars from parts & sub-assemblies provided.
The manufacturing of those parts & sub-assemblies was elsewhere and for cost-containing & logistics reasons, would be done in batches.
Side note: "Just-in-time" wasn't often practiced in the US at the time (1st introduced in Australia by BMC, though largely credited to the Japanese).
So, many if not most '68 Hemi cars had blocks cast in '66, as one example.
I'm sure that process varied among sub-assemblies (dashes for instance) owing to storage space, vendor procedures, and the need to keep the assembly lines flowing.
Places where raw ore came in one door of a facility and a finished car came out the other were & remain rare (Ford's Rouge megalith).