The Reagan administration is searching for a third-party Central American nation where U.S. military personnel could train Salvadoran forces, with neighboring Honduras the logical but by no means certain site.

Pentagon officials said yesterday that they want to train half the Salvadoran army, enough to station an infantry battalion in each of the country's 14 provinces, and they need a big training area for such a major job.

Using a nearby country as a training ground, they added, would stretch the $110 million President Reagan wants in military aid for El Salvador. They said it cost about $6 million in the past to train and equip one 900-man Salvadoran battalion at Fort Bragg, N.C.

Officials said a limited number of Salvadorans still might be trained at Fort Bragg and in Panama.

Honduras is tranquil compared to El Salvador, where army troops are battling guerrillas. Congress most likely would pose fewer objections to sending more U.S. advisers into a country like Honduras than into El Salvador.

Whether Honduras or any other country in the region will allow the United States to use part of its territory as a training ground for Salvadoran forces is the big question, officials said. The administration is sounding out Latin American governments, they added.

Pentagon officials sounded optimistic as they described plans for upgrading the Salvadoran army under an intensified training program which would be financed by the $110 million now requested. They said such training could leave the Salvadoran army highly effective.

The Pentagon plans to emphasize small-unit tactics, with 320-man outfits called cazadores, meaning "hunters," seen as the key for finding and killing the guerrilla forces in the back country of El Salvador.

Once the cazadores cleared a region of guerrillas and established a ring of protection, civic action groups would move in to dig wells, fix roads and build schools to win over the populace, U.S. planners said.

Reminded that those techniques were tried with mixed success in Vietnam, Pentagon officials said El Salvador is a much more manageable problem.

One reason, they said, is that El Salvador, at 8,124 square miles, is only about one-eighth the size of South Vietnam, 67,108 square miles. Another is that the guerrillas in El Salvador are not as tough a foe as were the Vietcong.

If Congress approves the requested $110 million, Pentagon officials said they would spend about $25 million of it to train "over 50 percent of the armed forces." Equipment would include new UH1 helicopters and trucks to give the cazadores more mobility.

Besides providing one cazadores light infantry unit for each of the country's 14 provinces, the Pentagon said the $110 million would finance training for troops who would stay in the provinces to maintain permanent security after the main guerrilla forces "had been neutralized."

The Pentagon plan also calls for increasing the 22,000-man Salvadoran army by 8,000, sending 1,000 officers, sergeants and enlisted specialists to military schools in Panama and training an additional 500 officer candidates.

All this training, said the Pentagon in a justification paper prepared for Congress, "could also have a positive effect on the human rights performance of the Salvadoran military and improve their command and control."

Breaking down the requested $110 million by percentages, the Pentagon said 37 percent would be spent on ammunition, spare parts and other "consumables," 25 percent to train 50 percent of the Salvadoran army, 25 percent for civic improvements reminiscent of the programs tried in Vietnam "to win hearts and minds," and 12 percent to buy helicopters, trucks and boats.

The Salvadoran army needs military help in a hurry, Pentagon officials said. Last week the army ran so low on ammunition that five C130 transports loaded with bullets for the Salvadorans' M16 rifles had to rushed to the country.

Asked why tiny El Salvador is important militarily to the United States, Pentagon officials said the country cannot be looked at in isolation. If its government is allowed to fall, they contended, other nations in the region will be targeted by Cuba's Fidel Castro and his Marxist allies.