Rep. William V. (Bill) Alexander Jr. (D-Ark.), who held talks last week with Cuban President Fidel Castro, proposed yesterday that the Democratic Party call for a conference of Western Hemisphere nations, including Cuba, as part of a policy to bring peace to Central America.

The idea of including Cuba in a dialogue is one aspect of a proposal that Alexander said he plans to offer other House Democrats as a possible party alternative to Reagan administration policy in the region.

Although Democrats have united in opposing Reagan's aid to "contra" rebels fighting the leftist government of Nicaragua, party members have produced few, if any, creative alternatives.

The Reagan administration has rejected a Cuban role in Central America peace negotiations on grounds that it would institutionalize Cuban involvement on the mainland and provide Castro with a new forum from which to attack the United States.

Alexander, deputy House majority whip, returned last Thursday from a six-day visit to Cuba during which he and two other members of Congress talked with Castro for a total of 37 1/2 hours.

They said he appeared "very conciliatory" and serious in his desire to "engage in an exchange of views" with the United States on Central America and on other issues involving the two nations.

"I think Castro has changed his foreign policy toward the United States in order to reach a settlement," Alexander said in an interview. "He said he would talk, that he would participate and cooperate with the United States in talks designed to achieve a political solution in Nicaragua and El Salvador."

Castro also said he was willing to discuss withdrawing Cuban troops from Angola as part of an effort to reduce tension with the United States, Alexander said.

Castro stressed that "he cannot negotiate for third countries," Alexander said, but "ratified his role in Central America. All the world knows he has a role. He is high priest of revolution there."

U.S. refusal to talk to him is "adolescent" and inconsistent with its continuing dialogue with the Soviet Union and the People's Republic of China, Alexander said.

Talking to Cuba "can't give to Castro a status in the world" higher than that he has achieved on his own, Alexander said.

Alexander's policy draft calls for "a general hemispheric peace conference which would focus on the major disputed questions of the Americas . . . all nations of the hemisphere who profess an interest in resolving the crisis should be invited.

"This would specifically include Castro's Cuba," it continued. "Realistically, there is little hope for a long-term peaceful solution in Central America without Cuba at least acquiescing in the terms of an agreement."

A State Department official yesterday reaffirmed the administration's no-talk policy, saying: "This is about the 45th time in the last three years Castro has signaled this kind of thing only to signal the opposite way later on. We've checked all of them out, and they're not serious . . . . We put no stock in this whatsoever."

Angel Pino, a spokesman for the Cuban interests section here, said Castro's remarks to Alexander were a "perhaps more vehemently stated" reiteration of Cuba's longstanding willingness to talk to the United States. "We want to collaborate in search of a peaceful solution to the problems of the region," Pino said.

Alexander agreed that this was the official Cuban line. "What Castro is doing is sending messages that would change the official line . . . . He's offering a new and different direction," he added.

Alexander's draft calls for strong support of the Contadora peace initiative of Mexico, Panama, Colombia and Venezuela, which have been talking with the five Central American republics and circulating draft treaty proposals. They plan to meet again in mid-February.

Rep. Jim Leach (R-Iowa), who joined Alexander and Rep. Mickey Leland (D-Tex.) in meeting with Castro, said in a separate interview that Castro said "very vehemently" that Cuba and Nicaragua would back any Contadora agreement and favor strong provisions to verify compliance with the agreement.

"I don't put too much store in words alone, but you're unlikely to have good actions if you have bad words, so I consider this a positive sign," Leach said.

Alexander's draft says: "We should support change through democratization and peace through diplomacy, seeking a regionwide system of economic cooperation and open trading markets."

U.S. interest in Central America is "so great that the selective use of military force should remain an option but should be avoided if at all possible, a last resort used only to counter a clear case of imminent Soviet or Cuban dominance throughout the region," it adds.

Alexander also called for "a significant foreign assistance program" to the region that could be jointly funded by other Latin and European nations. Participants should work toward a regional ceasefire, an end to arms imports and an international security force to guarantee peace during negotiations, the draft said.