This description was told to me by a former GM engineer who lives a few blocks from me.
When the TR's were built (as all GM full frame cars) The body and frame were built by Fisher body. Fisher painted the cars, installed the glass, misc wiring and much of the other body hardware.
The final assembly was done at the division plant (back when Chevrolet was Chevrolet, Buick was Buick, Pontiac was Pontiac, etc)or by the 1980's by the assembly line such as Pontiac, MI (since they would send different divisional cars based on the same chassis designation down the same line). The final assembly
included install of the drivetrain, interior parts, front sheet metal and the rest of the trim. Its hard to say how long it took GM since Fisher body would build the body and frame as previously mentioned and then store/stock pile them and then ship to the plant on an as needed basis. You have to take into consideration the amount of time it would take to ship the body over to the site of final assembly.
The interesting thing I have noticed with a lot of 60's model GM products is that the body# rarely coincides with the serial sequence. Also the body build date can drastically vary by several weeks or months from the final assembly date. I have never checked my own Buick, but it would be safe to assume that a lot of the extended build cars would be close in this respect since GM was trying to bang out as many as they could at the end. The 86 and earlier cars would probably have varying body/assebly dates as mentioned.
Most vehicles made today and built start to finish at the same site. Ford was the first one to do this.
At one point in time, the River Rouge plant in Dearborn would manufacture many pieces for the production of the respective car in house.
