The arrangement is based directly on Duke Ellington's original record from 1930, including clarinetist Barney Bigard's solo. It's almost an exact transcription.
The choir sings the "official" melody and lyric, but then follows up with a so-called vocalese, singing Bigard's improvised melody. That melody is based on the official melody, embellishing it at places, and ignoring it completely at others. The choir, correspondingly, uses the original lyric as much as possible, but compresses or expands it as needed: for example, instead of "since my baby said goodbye", the choir just sings "My baby, bye," reducing the lyric to its essentials as Bigard did with the melody.
There is one bit I added that wasn't on the original record: the eight bars of polyphony starting at bar 57. The three parts sing independently, with staggered entrances, and each filling in gaps for the others. Here is how it looks in "piano roll" view, showing the interplay between the voices:
The soprano is light blue; the mezzo-soprano is taupe; the alto is brown.
Greg suggested that the entire choir sing the mezzo (middle) part, with soloists on the outer parts. You can hear the soprano solo really clearly, though the alto is a bit buried.
Finally, one tiny detail I obsessed over is at bar 67, the "s" in "soul". It seemed important to stretch it out, so I notated it that way. It's not easy for 40 singers to match rhythm on this, but they did it.