BY THE time the cherry blossoms bloom again, the name Ian McKellen may well be in the running for both a Tony and an Oscar. Such chances alwasy are iffy, but so are rich roles. And the striking British actor happens to have two of them.

McKellen is all but unknown in the United States. Now starring in the National's previews of Peter Shaffer's "Amadeus," which opens Wednesday, he's just completed his first international film as novelist D. H. Lawrence in "Priest of Love." It's possible that both performances will propel him into the actors' stratosphere.

In England, he's been there for quite a time. Both Laurence Oliver and John Gielgud agree that McKellen, 41, has become their peer in the gret classical roles. Queen Elizabeth II last year made him a CBE (Commander of the British Empire) and he's won several times, England's major acting awards. He's played the great classical leads with the National Theater of Great Britain and the Royal Shakespeare Company.

Last week in New York, between rehearsals for "Amadeus," he talked about his 20 years' experience (starting at Cambridge University) and the demands of American acting. "It's very busy, you know, always in motion. That's as thought American actors -- ant their directors -- dont quite trust the words.

"My tradition is quite different, perhaps because of the classics. I believe in primary concentration ont the playwright's words: their meaning, their sound, their tone, their limitless possibilities, equivocations and conciseness. I've found it possible to barely move but to make an audience listen.

"Of course I'm anxious to be understoon, a problem I know English actors have here. What I'm aiming for here, I suppose, is a Walter Cronkite accent, clear and deliberate. He's as you might say, a marvelous communicator. No misunderstanding his words. I shal be hoping to do that."

McKellen's acting philosophy has been developing since he was 4, when he was assigned to play a Massachusetts Colony Pilgrim in his Lancashire town's amateur play.

"I was decked out in black, widebrimmed hat, buckled shoes, a very proper, solemn little boy. Someone I'd known all my life, dressed up as a warlike Indian, tried to reassure me that the fierce makeup on his face was all in make-believe. 'Don't be afraid of me, sonny,' he urged. 'All this is make-believe.'"

"Well, for heaven's sakes! I thought, 'Of course it's make-believe! Do you think I'm an idiot?"

"It's been rather like that ever since. I've know I was making believe to be someone else, but I've always been able to look at it from a distance, to see, from outside, out front, what I was doing, where I was. It seemed only sinsible then and, of course, it's the pure child in all of us which treasures make-believe.

"If something doesn't feel right to me in rehearsal, I close my eyes, turn my back to the house and envision what I'm doing from out front.'Oh, yes,' I think, 'two paces to the right. Or wait a beat here. Or raise an eyebrow? I try to see what I'm doing from outside myself.

"But at the same time you have to be open, open, open, totally free of yourself while you're making believe to be someone else. And you have to have gotten to know him very well, from inside out."

This ability -- to be and to see at the same time -- likely is the reason McKellen has captured the mantle of previous generations of the great British generations of the great British players.

His role as Antonio Salieri in "Amadeus" is mercurial and rich -- one-line, stand-up comic throw-aways swirling swiftly into long vocal arias redolent of the great Shakespearean characters he's acted. In england, the part of Salieri was played by the admired older actor Paul Scofield, who won all of last season's London awards. But Scofield preferred to leave the American version up to someonw else. The part fell to McKellen at an opportune moment.

"I'd played my Hamlet, Romeo, Macbeth, Richard II Edward II in England and there was no point in repeating, repeating and repeating. I'd been a founding member of the Actor's Company.

"We'd done Ibsen, Chekov, Fedeau and Iris Murdoch. Then I'd gone on to the experimental-minded Royal Court Theater for 'Bent,'script the American Martin Sherman presented as having been written for me.

"Later Richard Gere did the Broadway "Bent' and I had a film, "Priest of Love.' So when Gerald Schoenfeld of the Shubert Organization came to me with 'Amadeus' his timing was just right.

"I don't look on this part -- or even this year -- as some huge leap forward. I see it as just one further step in what I've been trying to do ever since I left Cambridge. One step at a time. You have to keep going up, growing. Salieri does demand bravura acting of the most complex sort, but that's what I've been building up to all my career, the technical assurance to do what Shaffer demands of his actors.

"When I was offered Salieri in American," he said, "I realized that I had to see the play in London.Soon as Scofield cam on, I almost kept my hands over my eyes because I didn't want to be influenced. But I had to see the play as a whole, as Peter Hall directed in [Hall is also directing the new American version]. This production is quite a novelty, an expensively staged -- 28 actors -- serious drama for Broadway, where musicals are likelier risks."

McKellen's first American appearance was in the Soviet three-character drama, "The Promiise," imported from London in 1967 with Eileen atkins and another Ian, McShane. McKellen returned to America for his Actors's Company visit ot the Brooklyn Academy of Music in 1974. After "Bent" he toured Europe in his oneman tour-de-force, "Acting Shakespeare," drawn from his various roles as well as the sonnets.

Its single performance in America last spring brought him close to Washington for the CHALFA Festival, the Charlottesville/Albemarle Foundation for the Cncouragement of the Arts. There, at a formal dinner in the posh Farmington Country Club, he startled the well-fed wealthy with a daring entrance, making his way round the tables rolling out the stirring lines of Henry V. So complete was hiis triumph that Charlottesville claims his as its own, and last week planned to send a delegation to "Amadeus."

After leaving the 45th-floor rehearsal room of Broadway's minskoff Building on one recent night, McKellen didn't go to his hotel to rest. He instead went off to a preview at Joe Papphs Public Theater of Rosemary Harris in "The Seagull."

That's the way Gielgud and Olivier were at McKellen's age, wholly consumed in taking in everything possible about acting and audiences, plays and performances. He's theater to his boot-tips, his blue eyes alert for life's mocking ironies. America will relish his commanding art.