"We'll throw out what I call that 'health-farm' set, replace it and the costumes, choose another song -- Warren and Dubin have hundreds of 'em -- and have a whole new Act I finale." Thus speaks David Merrick of the Broadway-bound "42nd Street" at the Opera House.

He, director Gower Champion and librettist Michael Stewart did virtually the same thing with the original Act I finale of "Hello, Dolly!" while it was trying out here at the National in the fall of '63. That shift cost Merrick $40,000 and turned a promising musical into a smash.

This time, thanks to inflation, the new Act I finale will set Merrick back at least $100,000. "It's a lot of money," says the veteran producer, "but when you already have $2 million riding on a show, you protect the investment." So out will go one of designer Robin Wagner's more elaborate conceits: The elaborate turntable gym with all those silvery bars on which a flimsily clad covey of beauties goes round and round surrounded by mirrored walls.

"For about 30 seconds," Merrick says, "It's diverting. But nothing else happens. It's boring. Furthermore, the space it takes up deprives us of what should be a spectacular entrance. We're going to think of something else.

"The worst thing that can happen in a musical intended only for amusement is to allow a minute of boredom, of nothing happening. Of course we -- Gower and Mike -- hoped it might work, but on opeing night we saw that it wouldn't. It will take some production playing every night on Broadway, a record no individual has approached anywhere. Now, after six years of movie-making, Merrick is back on the Broadway beat. Not merely back: "42nd Street" is the most expensive musical ever mounted. It costs in the neighborhood of $200,000 a week to operate, with 55 people on stage, 28 in the orchestra pit and scores more invisible backstage. There are 400 costumes. "Think," sighs a principal, "of the dry-cleaning bills!"

Merrick doesn't allow himself to think of such things. "I'm thinking only of how to create the sort of lively, lavish, frivolous musical I believe people have been missing. I think the musical public is fed up with those solemn ones and those tiny little ones of a half-dozen people, skimpy sets and squeaky orchestras. I think it wants what I call this -- a song-and-dance extravaganza.

"With musicals, I've always tried to touch the public pulse. And while I must admit I don't really know where the American musical is heading, I think it's gotten too pretentious. The story lines of the oldies strike young audiences as pat and stodgy. You can't help observing that this recent spate of revivals seems pretty old hat and at the box office it's not the sensational smash people claim. Some of that comes from shoddy productions. I won't name 'em but take a look at those flimsy sets and tatty costumes.

"It's interesting that although Richard Burton drew $421,000 in the first Toronto week of 'Camelot," that theater's capacity gross is $535,000. That's over $100,000 not beating down the doors. Doesn't that tell you something about these revivals?"

The lawyer who escaped from St. Louis to present nearly 90 productions on Broadway starting in 1954 is thinner now, but his black hair, mustache and dark eyes remain exactly as they are in the innumerable Hischfeld caricatures. dIn the six years since he deserted to the Beverly Hills Hotel, he has produced three pictures.

"For all the knocks it got, 'The Great Gatsby' has come out a winner, not a big winner but comfortably profitable," Merrick says. "Then there was 'Semi-Tough' and now the one that's just opened, 'Rough Cut.' That tells you a lot about movie producing, doesn't it? It's far more complicated, time-consuming and expensive than getting a Broadway production together. First you have to have a script that will attract a bankable star. In recent years, these have been men. I've gone after and gotten such bankable guys as Robert Redford and Burt Reynolds. They have choices stretching years ahead. Then you've got to get thogether the kind of staff and crew that's the cream. They're booked ahead, too. So it becomes a matter of time-juggling. The big, all-encompassing studios are gone. that's why three pictures in six years isn't a bad average.

"Inactivity, just waiting, makes me itch. I have a new picture in mind and from the days I'd do five or six shows a season. I couldn't do that now, with the most modest play budgeted around $300,000. I've never had that kind of money.That boils down to a million for three dramatic productions."

When he was retreating from his long run as Broadway's Abominable Showman, Merrick said: "I don't know where the money is any more. Joe Papp does -- grants and foundations. He knows where the money is."

Now he says, "That area is not anything like what it was. Joe's used his 'Chorus Line' profits to keep going, but they will be running out along with the foundation grants.

"What we don't have, among other things are theater producers. Sure, individual ones have their single hits. And a real veteran, Alfred de Liagre, had had a smash with 'Deathtrap.' Roger Stevens is more active -- has to be -- with the Kennedy Center, his everlasting glory, but Stevens and Whitehead don't produce the way they once did.

"What has happened is that the stop-clause is a thing of the past. That meant that if a show ran under a given gross one week, it would be forced out of a theater to make room for something new. Now theater owners are content to pay their bills with little profit, if any, to avoid the losses of being dark."

Merrick looks with amused compassion at the fued between the two major theater chains -- the Shubert Organization and the Nederlander firm. "I've never owned a theater, but of course I can understand them. Their ownership has turned them into active, investing producers. I've tried to think of myself as a creative producer. Yes, I've imported productions from London, changed some, too. If the Shuberts weren't importing 'Amadeus,' I'd have put in a bid for it, the best show I've seen lately in London. Not that I think it's likely to be a broadly popular success here. But it is a distinguished play and I'm proud of the many I did.

"What I've always enjoyed doing is putting things together so that they'd jell."

Now he's trying to do it again with "42nd Street." Besides such creators as Champion, Stewart, Wagner and young Mark Bramble -- once his office boy -- Merrick has around him such other longtime associates as Alan De Lyn, Helen Mickerson, costumer Theoni Aldredge, lighting whiz Tharon Musser, conductor John Lesko and Orchestrator Philip J. Lang. Used to working together, they're aware that things here have yet to be "fixed." CAPTION: Picture, "When you have $2 million riding on a show, you protect the investment."