Engine sub assemblies all bolt into the same four holes so as long as the pieces arrived at the right time there should be no delay to the assembly line.

The body arrived with the appropriate'differences' already in place.

If they tried to do them in batches it would throw a real wrench into things if there was a shortage of say the exhaust components, brake boost etc. It would be easier to pull one aside for these problems than a batch of them

I would think the only time we would see batches of similar vehicles was on the first and second day of model year production when company show cars etc were built and there wasn't the same committment to maintaining assembly quotas for the day.