Elvis Presley was either a gospel singer who sang the blues, or a blues singer who sang gospel. In either case, such songs, if they are authentic, are created through the "folk process" which means that each performer adds something of themselves to the song - sometimes a lot, sometimes just a bit. Elvis's radically transformed "Blue Moon of Kentucky" is the one you're likely to find in published sheet music, yet his version has an original verse to begin the song, a brisk R&B tempo, and many other significant alterations. The original waltz by Monroe is now relegated to history {though it's still the state song of Kentucky!}. And so on. Especially at Sun records, even on numbers that were either unreleased, pulled from release, or unpublished, Elvis had the creative freedom to reinvent music that he lost, by contract, to RCA. RCA instisted on "complete control" of the creative process. While they never quite got that, they certainly put the breaks on the young performer's creative development at a crucial stage.
Still, it was in gospel music and spirituals where he was most free to be as inventive as he chose {or to follow arrangements he loved}. This is why most people generally are familiar with his versions than earlier ones, which would probably dishearten Elvis.
I would say to just keep up your search: it's a great piece of music. As for other "Elvishimselvis"ed music {I loved that cute one when I saw it recently on some liner notes, too!) Another most intriguing number are his several very different versions of "Farther Along," a Sunday morning standard down south. His first version was said to be "dirgelike" in its slow, sad tempo, but they hadn't heard the last of it . . . One time, he actually "lined out" the song, which is a quite ancient technique in black spiritual singing, at first throwing the Sweet Inspirations, a black, female gospel group, totally off! At first they didn't seem to know what to do. After this, and a powerful version of Crouch's "Oh Happy Day," Elvis slyly remarked: "scared 'em!" The "Sweets" as they were known, were generally not southerners, but Elvis had attended genuine "Sanctified" church "revivals" and services as a 'tween in Tupelo, Miss. Sanctified churches are black, and he lived on what was then called "The Hill" in Tupelo, a black neighborhood with a few "white" houses generously so designated, by black landowners, for those whites who had been living in alleys and such - as were the Presleys. Though generally taunted and ostracized in school, he had friends at home, and was welcomed to sanctified revivals and services.
If anyone wants to know why Elvis Presley has such a vital place in American music, his unusual upbringing had much to with it.
If not for his contributions, I think few "mainstream" music fans would even KNOW of such songs. "Rock 'n' roll music," he said in 1968, "is really gospel, or rhythm and blues."