Special presidential envoy Richard B. Stone arrived in San Salvador last night on his second official trip to Central America, planning to meet with representatives of leftist Salvadoran rebels in an effort to bring them into the political process there.

Stone got his marching orders in a 20-minute meeting with President Reagan before leaving Andrews Air Force Base shortly after noon. White House spokesman Larry Speakes declined comment on Stone's plans for the talks with the left, saying only that Reagan had asked Stone "to continue the dialogue . . . to achieve economic stability" in Central America.

Stone's move came as maneuvering intensified on Capitol Hill in preparation for votes on Central American aid scheduled this month.

Stone will clear his plans with government officials in El Salvador before proceeding to the scheduled meeting Saturday, according to U.S. officials. Salvadoran sources who confirmed that talks are planned nonetheless disagreed on whether they will occur in Panama, Costa Rica or Mexico City.

In San Jose, Costa Rica, the combined leadership of the Democratic Revolutionary Front and the Farabundo Marti National Liberation Front of El Salvador issued a statement reiterating that it "is interested and willing to initiate a process of dialogue" with Stone.

"There are substantive matters for a dialogue with the government of the United States related to the solution of the Salvadoran conflict," the statement said.

But Stone's mandate, consistently repeated by both the State Department and the Salvadoran government, is not to participate in the discussions but only to "facilitate" talks between the guerrillas and the Salvadoran Peace Commission, which was set up to try to bring the leftists into elections scheduled later this year.

Salvadoran President Alvaro Magana insisted in Washington last week that any talks with the left be restricted to that issue. The guerrillas have proposed a six-point agenda, including elections, that would give them an active role in the government.

Diplomatic sources stressed that Stone hopes to widen the Salvadoran government's agenda to a point short of power-sharing by broadening the definition of requirements for leftist participation in elections. But commission President Francisco Quinonez insisted that no concrete proposal exists.

Stone's meeting will be "the first look at analyzing the problem, and we will not accept any foreign intervention," Quinonez said, apparently referring to direct U.S. participation.

In a related development, Salvadoran Judge Bernardo Rauda Murcia said he expects to receive police evidence this week. This would be another step toward beginning the long-delayed trial of five National Guardsmen accused in the 1981 murder of four American churchwomen.

He warned that new appeals might delay the case further and gave no trial date. The announcement came just two weeks before Reagan must certify to Congress that progress is being made in that case--as well as in bettering the human rights situation and in moving toward elections--as conditions for the continued flow of military aid to El Salvador. Reagan is pushing for additional aid to El Salvador in votes scheduled in both houses before the end of the month.

The House also is scheduled to vote July 14 on a proposed cutoff of all covert aid to rebels opposing Nicaragua's leftist government.