I've always felt ACL was an original work. While some of the stories and monolgues were drawn from real life dancers in the tape sessions, Kirkwood and Dante still had the formidable task of shaping all of that voluminous material into a coherent two hour narrative and creating 19 distinct characters, some based in part on tiny pieces of the life stories of various dancers, but most of whom were wholly original creations. All writers borrow from somewhere (their own personal stories or those of family members, friends, acquaintances, strangers they meet or from books, films, television shows that are both fictional and non-fictional that they encounter). I think you can only call a show an "adapted" work if the plot and characters (real or fictional) came entirely from another source (book, play, film or history). The book writers of ACL didn't have a plot with a beginning, middle and end when they began writing the show and so they had to construct one from scratch. They also didn't have any pre-set characters and had to create those as well (and even those who are sort of based on some of the dancers in the tape sessions ended up being amalgams of several different people, with a lot of fiction and dramatic license mixed in). Some of the story songs only took a line or two of dialogue from the tape sessions and the rest was invented by Ed Kleban. Similarly some characters like Cassie, Zach and Bobby (his monologue was completely the fictional invention of Neil Simon), aren't based on anyone.
All in all, despite some dialogue being taken from real life, I think that the book writers, lyricist and composer (and Bennett) created an essentially new work with ACL -- one that had too much original invention and was simply too much of a departure from the bits and pieces of stories in those transcripts to be considered an "adapted work."
"What a story........ everything but the bloodhounds snappin' at her rear end." -- Birdie
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"The Devil Be Hittin' Me" -- Whitney
