While this doesn't help with the OP's project, the linked thread does highlight that it may have never been possible to build a high end multi-speed derailleur racing bike with parts made in America.
A couple of interesting points in the article state that in 1965 the "Paramount was one of only two racing machines manufactured in America," but a quick look at the build reveals it wasn't close to completely American made parts. I do believe that you could buy a completely American made Varsity. It goes to show that despite the American aerospace and automotive manufacturing heritage, road racing bikes never really had the same prominence as their lower performing siblings.
https://www.bikeforums.net/classic-v...mount-p13.html
John