I've heard a few theories.
One was it was a 'day' code, but a 2-year cycle.
The other was it's the shift number, with #1 being the first shift of the first day of the production year, #2 being the 2nd shift of the first day, etc.
If you figure production started Sept 1964, and they worked 26 days a month with 2 shifts per day,
August would be about 572 shifts in, so your '599' is in the ballpark.
Were all Buick engines built in the same plant at Flint?