Concerning "We Shall Overcome," I'll quote some comments from James J. Fuld, The Book of World-Famous Music, 4th ed. (Dover, pp. 622-628):

The music and words of We Shall Overcome, the unofficial Negro freedom anthem which was given prominent recognition by President Johnson, are derived from a number of sources.
The simple melody of the first four bars of We Shall Overcome is almost identical with that of the hymn whose opening words are "O Sanctissima."
The words of We Shall Overcome seem to have had their source in an original hymn by C. Albert Tindley, entitled I'll Overcome Some Day,...
The first printed confirmation of the wedding of the above introductory music and the above title is in the Negro gospel song I'll Overcome Someday, published May 1, 1945, by Martin & Morris Music Studio, Chicago.

Masato