FOR A SHEER musical comedy thrill, you can't improve much on the galvanizing opening image of "42nd Street," with the curtain rising slowly to reveal a forest of flying feet.

They're back for a two-month encore at the National Theater in producer David Merrick's mammoth musical about backstage backstabbing among the showgals and guys tapping for "glory at 32 bucks a week." (March seems to be the month for living in the past -- the Kennedy Center has "Take Me Along" at the Eisenhower and the Gershwin musical "My One and Only" begins this Friday night at the Opera House).

Though some of the faces on "42nd Street" have changed since the last visit, it's still the same swell show, perhaps even a little more revved-up and self-aware.

It's certainly the same old story: At auditions, director Julian Mash gives neophyte hoofer Peggy Sawyer the hook ("Am-scray, toots!"). But Peg's persistent, and when aging star Dorothy Brock breaks an ankle before opening night, Sawyer goes out there a youngster and comes back a star, etc.

As Sawyer, Gail Benedict is "pretty hot stuff in the steps department," all-American and athletic, with a good slapstick style; and as perennial juvenile Billy Lawlor, James Mellon has springs in his shoes and helium in his terrific tenor.

Returning to Washington in the lead roles, Delores Gray and Barry Nelson are better than ever. As diva Dorothy Brock, Gray has loosened up and sings with more authority and power. And one of the night's highlights is watching craggy, shaggy Nelson attempt to force a smile through his lockjaw grimace in the role of Julian Marsh, who has given his all to Broadway, "that glittering gulch."

Though the performers dish up some deliciously corny lines, you can safely disregard the incidental script. The real stars are the glittery production numbers. The late Gower Champion's staging is a scaled-down homage to Busby Berkeley, with revolving turntables, an enchanting shadow dance and gobs of kewpie-faced gals with gorgeous gams and squealing baby-doll voices. And musical director Stephen Bates has brightened up the old-friend feel of the great Harry Warren-Al Dubin tunes like "We're in the Money," "Lullaby of Broadway" and "You're Getting to Be a Habit With Me."

"Guys, you gotta pick 'em up and lay 'em down," barks a moxie character at one point. "They're paying $4.40 a seat out there!" Though the cost has gone up (considerably), the "kids on the line" at "42nd Street" are still providing more than your money's worth. 42ND STREET -- At the National Theater through May 5.