Second your vote.
Bill Mitchell seems to have had a special relationship with Buick stretching back to the “Skylark” show car of the early 50’s (51 or 53—don’t recall exactly).
The Riviera of 1963 was Bill’s first shot at the new styling that was to be introduced across the GM product line by, say, 1966, which was also IMHO the peak of the Riviera’s stylistic development (as your photo illustrates).
That was the era in which he established himself once and for all as the heir and successor to Harley Earl.
Of course, my Dad was a Buick man from ‘51 to the end, and the family carried on this tradition for years after his passing.