The Western plan for a peaceful transition to independence here will be sent to the U.N. Security Council next week even if South Africa or the nationalists fighting against South African control fail to accept the proposal.

Ambassador Donald McHenry of the U.S. mission at the United Nations, in announcing decision here last night, said he is hopeful that the plan will be adopted and "implemented with dispatch," by the United Nations.

The plan involves a "substantial" U.N. civilian and military presence in Namibia (Southwest Africa), he added.

South Africa, which has administered this mineral-rich territory for almost 60 years under an old league of Nations mandate, had been negotiating indirectly with the Soviet-backed nationalist Southwest African Peoples Organization (SWAPO) over holding of U.N. supervised elections so Namibia can become independent by the end of the year.

Those negotiations have taken place through the auspices of five Western nations.

If the Security Council does adopt the Western proposal, an internationally acceptable solution will be set in motion, which could make it more difficult for South Africa to go ahead with its own "internal solution," involving elections without SWAPO. This could also make it more difficult for SWAPO to continue its 12-year-old guerrilla war against South African troops in northern Namibia.

Political observers here feel that both South Africa and SWAPO are hoping the other side will reject the proposals first, thus incurring the international onus.

The final western plan calls for a phased withdrawal of an estimated 20,000 South African troops, dropping to the level of 1,500 just before elections to establish a transition government. The voting is to be supervised by a civilian U.N. transition team and a peacekeeping force whose size is to be decided by Secretary General Kurt Waldheim.

As soon as the election results are official, South Africa is to withdraw the remainder of its forces, South Africa fears that the estimated 5,000 SWAPO guerrillas now in southern Angola will enter Namibia and, backed by Cubans, take over once South African troops withdraw. The U.N. peacekeeping force is supposed to prevent this, McHenry said.

Black African nations at the United Nationa are almost certain to heighten their calls for a total economic boycott against South Africa if it rejects the Western plan.

McHenry refused to say what concrete steps might be taken against South Africa if it breaks off the talks and goes agead with an internal solution. But local political leaders who spoke with McHenry said he told them the consequences would involve sanctions against South Africa and Namibia.

The U.S. envoy said he was hopeful that there would be sufficient support in the 15-member Security Council to get the plan adopted.

The black African countries which have been involved in the negotiations behind the scenes - the so-called frontline states - have given "their general support" to the plan, McHenry said. They reportedly are encouraging SWAPO to accept the proposal.

McHenry is visiting here to discuss the proposal with leaders of local political groups. He said he was encouraged by their agreement with the plan. It is hoped that such a favorable response will lessen South Africa's resistance. South Africa maintains it is acting on behalf of the people living in Namibia.

Local church leaders who enjoy a high degree of influence and respect, especially among the black population, told McHenry they endorsed the proposal. A coalition of centrist parties, the Namibia National front, also indicated it would support it. Most important, a coalition of more conservative parties, the Democratic Turnhalle Alliance which South Africa is backing, has said there are positive aspects to the latest proposal.

McHenry urged that the plan be accepted by Swapo and South Africa, adding If people want iron-clad guarantees, they better go to another planet.