Ronald Reagan has completed selection of his Cabinet nominees and will be able to announce all but two of them on Monday, informed sources said last night.

The president-elect's choice for secretary of housing and urban development is said to be Samuel R. Pierce Jr., a former New York judge and the first black to sit on the boards of directors of two major corporations.

Pierce, whose name had not come up in previous Cabinet speculation, visited Reagan on Thursday. That same day Philip V. Sanchez, who reportedly was Reagan's first choice for HUD, withdrew his name from consideration.

Pierce, 58, was general counsel of the Treasury Department during the Nixon administration, and has held a variety of business posts, including governor of the American Stock Exchange. He has not been identified with civil rights groups.

The announcement of two of Reagan's choices, including the nominee for secretary of agriculture, will not be made Monday because their security and conflict-of-interest checks have not been completed, sources said. s

Reagan settled on a nominee for secretary of agriculture after meeting in California on Thursday with Richard Lyng, a former president of the American Meat Instiute, and John L. Block, Illinois' secretary of agriculture. It was not known which man Reagan selected.

The withdrawals of Sanchez and earlier of Thomas Sowell, a conservative black economist who said he was offered a Cabinet post identified by transition sources as secretary of education, had frustrated Reagan's aides in their attempts to recruit minorities.

Former South Carolina governor James Edwards is expected to be Reagan's choice for secretary of energy. And Georgetown University Prof. Jeane J. Kirkpatrick remained the leading candidate for the Cabinet-level post of ambassador to the United Nations, according to transition sources.

Reagan's choice for interior secretary, James Watt of Colorado, was the subject of a surge of rumors yesterday that the president-elect was rethinking his choice in the face of a tough campaign of opposition from environmentalists. But this was denied by senior Reagan officials.

"We're still going strongly for Jim," one well-placed official said. "We're going to hang tough."

The head of Reagan's Interior Department transition team, Richard Richards, was said to be traveling to Colorado this weekend with a batch of transition briefing papers to help Watt prepare for Senate confirmation hearings.

Richards was traveling with not only good news for Watt, but also good news of his own. Reagan telephoned yesterday, according to an informed transition source, and formally told Richards he was supporting him to be the new chairman of the Republican National Committee, as previously reported in The Washington Post.

Watt has taken a number of stands against environmentalist interests and against benefits to the handicapped, aged, and underprivileged, in his role as the head of a legal action group.

Vice President-elect George Bush, meanwhile, has been assured of a seat close to the seat of power in the White House. According to informed sources, Reagan will continue the precedent set by his predecessor, Jimmy Carter, of providing the vice president with an office in the west wing of the White House, just down the corridor from the Oval Office, rather than housing the vice president in the adjacent Old Executive Office Building. Bush will move into the office presently occupied by Vice President Mondale, these sources said.

Meanwhile, transition team officials denied yesterday that Reagan plans to fire Gen. David C. Jones as chairman of the Joint Chiefs of Staff.

Questions about the general's future were generated by a Washington Star story asserting that Reagan's defense transition team had recommended dumping Jones and that Reagan himself plans to do this.

William Van Cleave, head of the defense transition team, told the Post yesterday in an interview in his temporary Pentagon office:

"We've made no recommendation and have not been asked for any recommendation" regarding Jones. "No one on my staff has sent any recommendation to me. I would know about it if any such recommendation had been made. We run a tight ship here."

Some conservatives have assailed Jones for his alleged softness on defense, citing his support of the Panama Canal Treaty and the pending arms control treaty known as SALT II.

James Brady, spokesman for the Reagan transition team, said that "I know Ronald Reagan has made no decision regarding Gen. Jones. I might add it is a term appointment, like the FBI." Since the Reagan team intends to allow the FBI director to finish his term, some observers viewed Brady's comparison as a solid indication that the president-elect would allow Jones to finish his two-year term as well.

Jones was named to a term that began last June. The Senate Armed Services Committee confirmed him on a 12-to-1 vote, with only Gordon Humphrey (R-N.H.) against him. An aide to Gen. Jones said yesterday that the chairman of the Joint Chiefs intends to finish his two-year term and will not resign voluntarily. However, Jones does serve at the pleasure of the president and thus could be fired by Reagan. A number of senior congressional staffers have advised transition team members against this course, arguing it would set a bad precedent of politicizing the Joint Chiefs of Staff.