The original diagram from the 1953-1982 parts book last published in 1984 and shows for the most part how the birdcage and windshield frame were assembled together.
When available from GM, the A pillar came as one large section and the lower reinforcement number 7 was assembled together with the inner and outer sections spot welded.
The lower A pillar as to this date is not available!
This picture is from the 1984 GM parts book,
This picture is from the 1972 GM parts book.