When it comes to specials, nothing can match a Radio City Music Hall spectacular. Darlin' Sandy Duncan and her numerous colleagues demonstrate the point in a flashy, opulent new extravaganza entitled, ''Five-Six-Seven-Eight . . . DANCE!'' which has just opened in a burst of summery airs. As directed and choreographed by Ron Field, reveling in the Music Hall's elevators and other mechanical wonders, the show proves itself right down to the exclamation point.
''Five-Six-Seven-Eight . . . DANCE!'' indulges extensively and at times charmingly in nostalgia, as it reprises a whole range of tunes and styles from America's pop album. The show opens with triple projections of backstage scenes of dancers warming up and the stage manager alerting the cast. The informal atmosphere established, Miss Duncan, the Rockettes, and attendant singers and dancers get things moving with the title song and other ditties. Except for intermission, the pace seldom slackens for the next two hours.
Early on, Miss Duncan introduces Don Correia, her accomplished singing and dancing spouse, in a slightly coy but amusing sequence involving the Rockettes. Mr. Correia performs Rodgers and Hart's ''You Mustn't Kick It Around,'' while he and Miss Duncan make a pretty duet of Irving Berlin's ''It Only Happens When I Dance With You.'' The reassuring Rockettes back up Armelia McQueen in her protest at not being allowed to dance.
Although the accent throughout is on nimble feet and high stepping, the musical melange, smartly conducted by Thomas Helm, features pop and musical-comedy favorites from across the generations. For scenic super-spectacular spectacles, there are a World War II battleship sequence, complete with main gun turret, recalling a Rockettes performance in New York Harbor for returning sailors, and a jet-plane flight in which everybody, but everybody, seems to be flying down to Rio.
For sheer charm and grace nothing surpasses the brief but exquisite appearance of Marge Champion. Following family-album pictures of little Marge and film clips of excerpts from those wonderful Marge and Gower Champion partnerings, Miss Champion has a musical exchange with Miss Duncan and then dances beautifully with Mr. Correia.
Not everything works as well in ''Five-Six-Seven-Eight . . . DANCE!'' The great and glorious clown-dancer-mime Bill Irwin cavorts all too briefly and incidentally. On the other hand, the show squanders valuable minutes on Ken Sacha's mildly vulgar impersonation of Bette Midler. Miss Duncan's fleeting ascent in Peter Pan costume is spectacular but slightly irrelevant even though she sings ''Neverland'' from the Broadway musical in the revival of which she starred.
On the whole, however, credit Mr. Field with having devised a crowd pleaser with class. He has been ably assisted by composers David Zippel and Wally Harper , orchestrator Bill Byers, and Mark Hummel and Donald York, who did the dance arrangements. Bruce Vilanch wrote the continuity. The extravaganza was designed by Tom H. John (scenery), Lindsay W. Davis (costumes), and Richard Nelson (lighting). Christopher Dixon created the highly effective film sequences. ''Five-Six-Seven-Eight . . . DANCE!'' will be tripping its light fantastic at the Music Hall through Sept. 5.