The American Symphony Orchestra, founded 20 years ago by Leopold Stokowski, has been conducted by such greats as Karl Boehm, Igor Markevich and Andre' Previn. The Westminster Symphonic Choir, too, has grown used to superstars at its helm, including Arturo Toscanini, Bruno Walter and Eugene Ormandy.

Thursday night, in the splendor of Avery Fisher Hall in New York's Lincoln Center, before an invitation-only audience of about 2,700 that includes Metropolitan Opera music director James Levine, violinist Isaac Stern and former British prime minister Edward Heath, the orchestra and chorus will perform Gustav Mahler's monumental Symphony No. 2 under the baton of Gilbert Kaplan.

Gilbert Kaplan?

How did armchair-conductor Gilbert Kaplan succeed in adding his name to such distinguished lists?

The answer, in short: money, chutzpah and an exhaustive effort.

The 41-year-old New Yorker, the millionaire editor and publisher of Institutional Investor magazine, planned the concert to celebrate the 15th anniversary of the founding of his magazine as well as to play out his musical fantasy. He has spent the past year actively preparing for the event, after which he will take 2,500 of his guests to dinner in the ballroom of the New York Hilton.

"I'm certain I will not come a cropper," said Kaplan, whose musical training before this project was limited to three years of piano lessons as a child. "I have absolutely no illusion that I'm a budding, great conductor, but this isn't an ego trip. The driving force for me is not conducting, but this particular piece of music."

Critics will not be given an opportunity to evaluate Kaplan's conducting. "We have a friendly understanding that no professional critics will be allowed," a representative of the chorus said.

And Kaplan declined to disclose the total cost, but he said one estimate of $100,000 for production costs "isn't too far off the mark." Fantasy, it seems, does not come cheap.

The symphony, known as "The Resurrection" and a major undertaking for even the most experienced conductor, takes about an hour and a half to perform. Vintage Mahler, it is full of Sturm und Drang.

In addition to an oversized orchestra and chorus, the score calls for two soprano soloists and two offstage brass ensembles. Until the advent of closed-circuit television for the brass, the symphony often was performed with three conductors.

Kaplan said one prominent conductor, whom he declined to name, dismissed his plans as "a Piper Cub pilot trying to fly the Concorde." And when he told Zubin Mehta he was going to try the Mahler, he said Mehta's response was simply, "I'm glad you said 'try.' "

He retained a tutor, a struggling young conductor named Charles Zachary Bornstein, and the two of them holed up for all of August last year at Kaplan's place at East Hampton. "We worked every day, nine hours a day, before I went to do those readings with the symphony," Kaplan said .

Eventually, after a prodigious effort, Kaplan did memorize the entire piece, each and every part in each and every measure.

He then turned himself into a globe-trotting Mahler Second freak, attending rehearsals and performances in Denver, Detroit, New York, Montreal, Amsterdam, Vienna, Tokyo and Melbourne.

He seized every opportunity to discuss the work with conductors. Many of them turned out to be quite generous with their assistance.

Last July, he flew to London on the outside chance that he could get invaluable advice from Sir Georg Solti, whose recording of the Mahler Second with the Chicago Symphony had just won a Grammy Award. "I was led to believe I could count on five or 10 minutes," Kaplan said.

He was received beside the pool of the maestro's London home, where Solti listened impatiently as Kaplan spent five minutes describing his project. All of a sudden, Kaplan said, Solti stopped him, saying, "That's all very interesting, but we have to get inside and get to work."

They spent the next two hours at Solti's piano. "He played and I conducted," Kaplan said. "It was the most stupendous two hours I have ever spent in music. We had a chance to go through the entire score.

"He liked most of what he saw, probably because my interpretation was largely his. I'm certain I've listened to his recording more than anybody else in the world."

Kaplan said he had another memorable session last January in Denver, where the Mahler Second was about to be conducted by Leonard Slatkin, the visiting music director of the St. Louis Symphony.

During an hour break between afternoon and evening rehearsals, Kaplan said, Slatkin "gave up dinner and went through my score, going over the pitfalls and telling me what he does to get through them."

Slatkin, reached earlier this week by phone in St. Louis, recalled that Kaplan could read music -- "but barely. In order to point out a passage I thought was difficult, I'd sort of have to sing it for him so he would recognize it.

"Still, since this is a one-shot-only thing, I don't think any professional musician can consider it a serious threat.

"He knows how the piece goes," Slatkin continued, "and, more important, he knows how he would like it to go. It's not impossible that he might have something worthwhile to say about it."

Slatkin, who had to decline Kaplan's invitation for Thursday night said, "I'm really curious to see how it turns out."

Kaplan flew here from his summer home in East Hampton today for his first rehearsal with the 200-voice chorus, the nationally renowned Westminster Symphonic Choir.

He'd already had several New York rehearsals with the 119-member American Symphony Orchestra, which makes its home in Carnegie Hall.

At the conclusion of today's rehearsal, Kaplan received spirited applause from the chorus, which is comprised of students at Westminster Choir College here. "He does it well," commented one of the tenors, "though it could be a little more fiery."

"I'm fiery enough when I conduct the recording," Kaplan said, adding that he plans to be more forceful in the two choral rehearsals next week.

Natalee Rosenthal, an assistant to the college president, said, "Of course, we were at first a little dubious about it."

Kaplan's public-relations representative contacted the college last December "with the proposition that his client was prepared to do the Mahler Second and had already engaged the American Symphony for the project," Rosenthal said.

Since the chorus already was scheduled to do the work a few months later, with Zubin Mehta and the New York Philharmonic, "the timing was okay," Rosenthal explained. "So I spoke with Dr. Flummerfelt."

Joseph Flummerfelt, director of the chorus, met with Kaplan and found him serious, she said. A short time later, Westminster Choir College president Ray Robinson signed the chorus' standard, all-expenses-paid contract for the engagement.

"The important thing," Rosenthal said, "is that we wouldn't have done it for any amount of money if we didn't think the man is sincere and capable and not just a wealthy dilettante. And we do think it will be fun for the young people."

Although Kaplan is a member of the Carnegie Hall board of directors, he had encountered somewhat more resistance in lining up the American Symphony, a democratic body whose members control all artistic decisions.

"We'd turned down similar things," recalled Arthur Aaron, manager of the orchestra. "The question of musical integrity has to be paramount here."

Before the orchestra agreed to a contract, Aaron said, Kaplan was required to appear twice before the full orchestra at unrelated rehearsals, "to prove to me he had the musical and technical conducting facilities to do it."

Until those two "readings," Kaplan acknowledged, he himself "had no idea if I could do it."

The booking offices at Lincoln Center and Carnegie Hall said they did not know if any amateur conductor had ever attempted such an ambitious project.

Aaron said Charlotte Bergen, a wealthy, semi-professional musician from New Jersey who died last month at the age of 84, had hired and conducted the American Symphony "three or four times a year for nine years." Yet her free, Saturday afternoon concerts at Carnegie Hall "never included anything as difficult as the Mahler Second," he said.

Aaron said he thinks Kaplan "will do a good job. There will be eight rehearsals, twice the normal complement, and he's already won the confidence of the orchestra."

The American Symphony has done worse with some professional conductors, many of whom, Aaron said, "are both incompetent and abusive."

"It's everyone's dream to conduct it," said Peter L. Ratner, a New York free-lance writer who specializes in articles on the German Romantics. "It's wonderfully metaphysical and grandiose, and any conductor's got to have quite a lot of chutzpah to do it."

Ratner, who also works as an investment banker for Drexel, Burnham & Lambert, paused for a moment and added, "I've always wanted to do it, too."