Ellen Barkin, Stephen Spinella, Jayne Houdyshell and Kate Shindle round out the cast of the one-night-only Broadway reading of “8,” the new play by Dustin Lance Black.Barkin steps in to replace Marisa Tomei, who recently withdrew due to a film commitment, according to reps. Barkin recently worked with “8″ helmer Joe Mantello when she co-starred with Mantello (also an actor) in the recent Broadway revival of “The Normal Heart,” for which the actress won a Tony. In the play assembled from reportage, transcripts and interviews, Barkin will appear with Christine Lahti as a lesbian couple among the plaintiffs of the case that challenged Proposition 8, the amendment that nixed same-sex marriage in California. Matt Bomer and Cheyenne Jackson have already signed on as a partners who were also part of the contingent of plaintiffs. Spinella (“Angels in America”) will play one of the witnesses at the trial, along with thesps including Anthony Edwards, Rob Reiner and Larry Kramer. Houdyshell, currently on the Main Stem in “Follies,” portrays Maggie Gallagher, a high-profile opponnent of gay marriage, while Shindle (“Legally Blonde”) is on hand as a clerk. Starry cast also includes Morgan Freeman and John Lithgow, appearing as the attorneys for the American Foundation for Equal Rights, and Bradley Whitford as the lawyer defending Prop. 8. After the play’s world preem in a fund-raiser event at Broadway’s Eugene O’Neill Theater Sept. 19, show will be seen around the country in an array of separate reading events at universities and community orgs.