Just making sure you are aware, but stock a '63 has a Dynaflow, and a '64 has an ST400. Both are switch pitch, but totally different driving experiences. If I had to do it over again, I would've avoided getting my '63 with the stock Dynaflow. Kinda neat, super smooth, but sluggish, slow, and a nearly never ending leak machine.
https://www.jalopyjournal.com/forum/threads/1964-buick-riviera.1165058/ States th400, good price though..