In 18 years of researching 64-72 Chevelle trim tags I've never found any true correlation between stamped week letters and actual days of a month from any plant. More often than not when one scheme may for two months, it invariably blows up the third. The camaros.org has a good description of how Camaros were built from 67-69. No, it's not Chevelle plants but it does represent both the Fisher Body plants and Chevrolet assembly plants of the time and how each side of the plant scheduled production.
As for engine assembly dates and when they were installed in the car, I've seen anywhere from a couple of days (even on the west coast) to over 3 months. Plant inventories didn't conform to a first in-first out basis. Engines, and other parts, arrived almost daily and were stacked for use. A car with a few days separation could have just had newly arrived engines spirited directly to the assembly line while older engines were still sitting in the racks. In 1968, plants were averaging anywhere from 200 to over 700 Chevelles a day (if one is to believe the released GM monthly VIN reports).
First-Generation Camaro Research and Restoration Data