Rick,
I'd say it's really on the cusp. Most passenger car plants made their model changeovers in the 2nd or 3rd week of Aug. And with the 65s and 66s being so similar, that model change was probably done in a week. With that many of those engines being installed, I'll bet the engines reached the assembly plants in less than 2 weeks after assembly.
Too bad that manifold isn't for an engine about 13 cubes bigger...............
Verne