Well, I finally found my cars build sheet. It was on top of the gas tank. It's in pretty bad shape, but I can read some of it. It looks like my '73 has been a lifelong Kansas City car. It was build at the Fairfax plant in Kansas City, Kansas. It sold new at Don Stein Buick in Overland Park, Kansas.
For those who are interested, GM still pruduces Chevy's and Buick's at the Fairfax plant, however it is a different building, in a different location than the plant where the Centurions were made. The original plant closed back in the mid 1980s.
My frame is done. It's blasted, painted, and new brake/fuel lines run. The engine and tranny are back in. All new brakes. It's time to put the body back on the frame!