When Nicaraguan junta leader Daniel Ortega made his government's maiden speech to the summit of nonaligned nations in Havana in 1979, he surprised the other delegates by spending a lot of his time attacking former senator Richard (Dick) Stone.

He said that Stone, a conservative Florida Democrat, had intervened unacceptably in Nicaragua's affairs by criticizing its friendship with Cuba and loudly questioning U.S. aid to the new Sandinista government.

In one episode, according to witnesses, Stone apparently antagonized Ortega earlier that year by confronting him and two other Nicaraguan officials in a hallway outside a Senate Foreign Relations Committee hearing room to press a point about Cuba.

Never shy, Stone has been antagonistic to leftist viewpoints in Central America--working for a year as a registered agent for the rightist government of Guatemala--and his impending nomination to be President Reagan's special envoy to El Salvador has become very controversial.

Administration officials said last night that Stone, 54, would be nominated today as the negotiator on which Rep. Clarence D. Long (D-Md.) had insisted as a condition for approval of increased U.S. aid to El Salvador.

But critics complained that Stone's work for the government of Guatemala would made it hard for him to deal effectively with the leftist guerrillas attempting to overthrow El Salvador's government.

"If people in the White House think that a person who has represented governments like Guatemala can move the peace process ahead, it is a real tragedy," an aide to Long said. He stressed, however, that Long would support whomever the president chose.

At the State Department, where officials had told the White House that special envoys by tradition are nonpolitical, career diplomats of ambassadorial rank, one official said Long "had done what he can to create a reasonable basis for compromise, but the administration is blowing it."

While one Republican senator said there would be a battle over Stone's nomination, congressional aides said he was likely to be supported for confirmation by the Foreign Relations Committee, where he was regarded as an effective problem-solver.

"Yes, he's controversial," said a senior administration official. But, this official added, "we think he'll do a good job and that he'll be confirmed."

Stone said in an interview last night that "In response to questions about a number of rumors, let me say that I never worked for United Fruit or United Brands, nor for any private groups or organizations from Guatemala. I never worked for or represented any Somoza interests or person in any way whatsoever," he added, referring to former Nicaraguan dictator Anastasio Somoza.

"My work representing the government of Guatemala was for two assignments for approxmiately one year ending early in 1982," he said. "The assignments were trying to organize a peace treaty with neighboring Belize and trying to arrange an improvement in human rights as a way of bettering U.S. relationships. After a year, we couldn't make enough progress on this, and I resigned."

Stone, a multimillionaire Miami lawyer, emerged in February as the administration's special representative for public diplomacy in Central America. From March, 1981, to March, 1982, he was registered as a foreign agent for the government of Guatemala, along with other members of his law firm, Proskauer, Rose, Goetz and Mendelsohn Inc., of New York and Boca Raton.

Surprised by his 1980 primary loss in his bid for a second Senate term, Stone blamed it on his vote favoring the Panama Canal treaties. He then worked on Reagan's foreign policy transition team.

He later lobbied to be named deputy secretary of state. But instead, he was offered secondary posts as assistant secretary of state for Latin American affairs or ambassador to the Organization of American States. "We didn't quite come to an understanding" on those jobs, Stone said.

Since his electoral defeat, Stone has been virtually invisible in Florida. In September he joined the new Washington affiliate of the Capital Bank of Miami as vice chairman of the board of directors, and has retained offices here while traveling back and forth to Central America on administration business.