Then there are always the exceptions. My '69 RS was built in Norwood in mid Jan 1969, the rear end was assembled at the Detroit Gear and Axle Plant on October 18, 1968 - 3 months in advance.
My other ride is a '66 Chevelle which has been in the family since day 1. I have the POP and sales documentation for this car. Although it is not a 1st Gen Camaro, it is interesting to note that for this car as well the rear axle was assembled much in advance of the actual assembly of the car itself. The rear axle for the Chevelle was built in Oct 1965 and the car was assembled in Flint, MI in mid March 1966 - a full 5 months in advance of the final build
As JohnZ stated usage usually occurred within a few days of component assemblies. I would be very interested if anyone has a rationale for how/why these exceptions did occur. Documentation clearly shwos that they did - but why?