Some Chevy parts books show a component part date code that spans two calendar years for a given car model year. For 1963 models, the Julian (day of year) calendar would begin at Jan. 1, 1962 with 001. The end would be on Dec. 31, 1963 with 730. Parts would have typically been manufactured from about July, 1962 through July, 1963.
There's still problems with this numbering scheme, as 666 would indicate very late Oct, 1963 (well into the '64 model year). If Buick somehow ran a 3-year date code sequence on engines beginning Jan. 1, 1961, then 666 would fall in late Oct, 1962. Wish we had the body plate to look at its date code for comparison.
Sometimes a factory paper build sheet can be found in the rear seat springs or under the carpet.
What has been, can be again. (Bob Wills, 1942)