The man most likely to be elected provisional president of El Salvador this week said today he would be "a good administrator" rather than a "balancing power" against the right wing and would just as soon not have the job.

Alvaro Alfredo Magana, 56, a prominent banker and attorney, was all but hand-picked for the nation's highest post by the Salvadoran armed forces and the U.S. government as an independent moderate acceptable to the U.S. Congress, which must finance President Reagan's determined policy to defeat leftist rebels here.

But Magana made it clear in an interview with journalists that he does not intend to do battle against the new, rightist-dominated Constituent Assembly or against its president, former Army major Roberto D'Aubuisson, whom the State Department once called a right-wing terrorist and banned from the United States. Rather, Magana said, he will subordinate himself to the new assembly's decisions.

"The only thing a provisional president could do is make small adjustments and try to administer well, because another group of people, the assembly, will be writing the legal framework," he said.

Magana (pronounced Ma-GAN-ya) said he expected "not conflict, but differences of opinion" between D'Aubuisson and himself "that will have to be overcome."

Magana called D'Aubuisson "an intelligent man" and said he did not agree that D'Aubuisson's Nationalist Republican Alliance party, called ARENA, and the party in the power-broker position, the National Conciliation Party, are extreme rightist groups.

"Some groups in some parties, maybe, but most of the leaders in the parties are progressive people," he said.

If Magana behaves as submissively toward the assembly as he says he will, the balance of power could shift decidedly to the far right that dominates the assembly, and especially to the National Conciliation Party, without whose votes neither D'Aubuisson's ARENA nor its ideological enemy, the ruling Christian Democratic Party, can legislate.

None of the three parties has a majority in the 60-member assembly, which was elected March 28 to write a constitution, name a provisional executive and set up full presidential elections. Magana said those could come early next year.

So deep are the parties' differences that the parties negotiated unsuccessfully for nearly a month on how to share executive power, exhausting the patience of both the influential armed forces and the Reagan administration, which cared less how the government looked here than how it looked to Congress.

Special presidential envoy Gen. Vernon Walters let the armed forces know here last week that continued U.S. aid was contingent on formation of a government of national unity that would safeguard land and economic reforms, observe human rights and fight the guerrillas.

"Help us to help you," he was reported to have said. The Army then suggested strongly in formal meetings with the politicians that Magana would be an appropriate choice as a leader.

Members of ARENA say the affable, scholarly-looking Magana is far too leftist and may still oppose his election. Magana, who received a master's degree in economics in 1955 from the University of Chicago, where he studied under the conservative economist Milton Friedman, rejected the leftist label and said he was "sure D'Aubuisson does not share the views of many of the people in the party."

Magana denied reports he was an adviser to exiled former junta member Gen. Alfonso Majano. Majano's reforms two years ago included kicking D'Aubuisson out of the Army.

But Magana admitted his Army connections brought him his new fame. Three former military presidents are his close friends, he said, and the Salvadoran Mortgage Bank, where he has been director for 17 years, has always handled the armed forces credit institution that provides low-interest loans to Army officers.

"They thought many years ago that I would be a good man for president," Magana said. Army officers proposed him as a possible future president in 1966, he said, "but if it doesn't come, I'll be very happy. I am not a politician."

Magana was born in the western provincial capital of Ahuachapan Oct. 8, 1925, and became an internationally recognized tax expert, working at the Organization of American States to help implement the Alliance for Progress in the 1960s and later as a treasury undersecretary here. He and his wife, Concha Marina, have six children, three of whom are studying or working in the United States.