[RA1John]

PMD engine plant cast, machined, assembled and painted the engine.

The vehicle assembly plant "dressed" the engine...
Alternator
Starter
Coil
Fan
AC compressor
PS pump
Engine mounts

More likely that the engine plant held back an engine than the vehicle plant, though this was possible too if there was an issue.
Worse case scenario would be partial VIN stamped onto block ('68+), installed into vehicle, then discovered to have an issue requiring a new engine assembly. Vehicle plant repair wouldn't be changing engine internals. They'd pull another engine per Broadcast and stamp that with correct VIN.

Theoretically, it would possible for en engine to go from casting to install within a few days, especially if it went into the Pontiac assembly plant.