Producers of the strong-selling Broadway revival of “The Best Man” aim to maintain the show’s critical mass of star power by tapping Cybill Shepherd, John Stamos, Kristin Davis and Elizabeth Ashley to join the production when some of its original cast members depart.Shepherd and Davis make their Rialto debuts in the show, filling the roles currently played by Candice Bergen and Kerry Butler, respectively. Stamos, who’ll step into the part now played by Eric McCormack, has appeared on Broadway in tuners including “Bye Bye Birdie,” “Nine” and “Cabaret.” Main Stem veteran Ashley takes over for Angela Lansbury. Original cast members James Earl Jones (nommed for a Tony for his perf in the show), John Larroquette and Jefferson Mays will stay on with “Best Man,” as will Mark Blum, who replaced Michael McKean after an injury sidelined the thesp. Storyline of the 1960 Gore Vidal play centers on two presidential candidates (played by Larroquette and, in the new casting, Stamos) and their backstage wrangling at the national convention of a political party. Although overshadowed at the B.O. throughout the spring by the mega-selling revival of “Death of a Salesman,” “Best Man” has nonetheless logged robust sales powered in part by the strength of a cast full of recognizable names. Show even managed to crack the top 10 of the weekly sales chart, an unusual feat for a nonmusical. Response to the production had already spurred an extension of the show’s limited run to Sept. 9, when it remains skedded to close. Shepherd, Stamos and Davis join “Best Man” July 10, while Ashley comes aboard July 24.