Willie Stargell trades his Pittsburgh Pirates uniform for white tie and tails tonight as narrator for the world premiere of Joseph Schwantner's "New Morning for the World" at the Kennedy Center.

The long-hitting first baseman--whose team defeated the Baltimore Orioles in the 1979 World Series--will read excerpts from speeches by Dr. Martin Luther King Jr. Schwantner, who won the Pulitzer Prize for composition the year the Pirates won the series, says the King texts were selected to serve as an emotional "trigger" in his music, which will be performed by the Eastman Philharmonia.

Stargell was chosen as narrator before Schwantner was selected as composer. Robert Freeman, director of the Eastman School of Music in Rochester, N.Y., watched the '79 Series and was impressed by how Stargell used that athletic event "as a platform for his views on community pride in Pittsburgh, his admiration for Dr. King and the Sickle Cell Anemia Foundation, which he heads."

The composer, who met with the baseball player in the early stages of composition, reported that Stargell's sense of humanity, resonant voice and imposing posture struck him "like a thunderbolt."

Stargell's recollection is less dramatic. "I remember we liked each other, but basically he did the talking and I did the listening."

Stargell, 41, retired from baseball after last season. For the past month he has been rehearsing in Rochester with the Eastman orchestra, using tape recordings to get his timing and his narrator's voice right. After a long career facing pitchers in clutch situations, he says he has no fear of stage fright.

"You work hard to do something well, and you are excited about doing it," he said yesterday on the phone. "I don't agree that there is such a thing as stage fright. There's nervousness and there's excitement, but there's confidence. When the moment comes, you take a deep breath and it should be natural."

The championship Pirates had a theme song, "We Are Family," by Sister Sledge. Stargell liked it for its "good words and nice beat." "New Morning for the World," he says, is "awesome."

Naturally, he's much in demand for product endorsements. Recently he turned down an offer to promote a pork product on television for which the fee was $500,000 for a two-year contract. "I know people are going to eat pork, but I'm not going to tell them to. It might not be the best diet for a child."

He agreed to be part of the Eastman program because of the King readings, but he does not think of his role as one designed to encourage minority participation in a premiere of contemporary symphonic music.

"There's no reason this should appeal necessarily to black people. Dr. King's stand went far beyond that. It you listen closely to him he's talking about all humanity."

Does he find Schwantner's composition a suitable musical reflection of King's ideas? "I don't try to compare the two. Dr. King was a human being, and this is music. But music creates a feeling, and Dr. King created a feeling, too."

Stargell will stand at a corner of the stage to narrate tonight. He says conductor David Effron is at the center, "and welcome to it." After the performance here--on a program that includes the premiere of a new work by Washington-born George Walker and music by Aaron Copland, Samuel Barber and Walter Piston--the orchestra goes on the road to Philadelphia, New York City, Pittsburgh and Rochester.

After that, Stargell says, "the thing that strikes my fancy is media work. Doing some sports commentating on television."

Stargell has been working with a voice coach in Rochester. Asked what the coach had done to improve his already authoritative baritone, Stargell politely declined to answer. "That's his business. He's a voice teacher. If I gave away what he does, maybe people wouldn't need him anymore. I'm new to show business. I don't want to violate any theatrical codes."