There are several numbers called "A Hundred Years From Now." The Byrds, Travis Tritt, even Johnathan King, amongst others have had their own particular version. A stage version became popular in the 1930s, but perhaps the best known (without the "I Won't Care" addition) was by Carson Robinson in about 1943. The title is also popular with poets. However, the OP here was interested in the number associated with Flatt and Scruggs, which is quite a different song, and therein lies my own interest, too.