New Jersey Gov. Thomas H. Kean met privately late today with Nicholas F. Brady, but refused to confirm or deny that Brady is his choice to fill the U.S. Senate vacancy created by the resignation of Harrison A. Williams Jr.

If appointed, Brady, 52, the chief executive officer of Dillon Read & Co., the New York investment firm, would take the Senate appointment only as a caretaker, according to Republican officials.

Kean could name the winner of the Senate race to replace Brady right after the November election, which would give him or her about two months' seniority over the other newly elected freshmen.

Carl Golden, Kean's press officer, not only declined to confirm or deny reports of Brady's appointment but warned that anyone who said the appointment has been decided "risks getting egg on his face."

However, Vice President Bush, through a spokesman, confirmed that he and Brady had discussed the Senate appointment.

Brady was co-chairman of the Bush For President committee during the 1980 New Jersey GOP primary.

Golden would not say where the Kean-Brady meeting was held other than at "an undisclosed location in northern New Jersey."

Kean declined to name either of the two leading candidates for the Republican Senate nomination in the June 8 primary--Rep. Millicent Fenwick and Jeffrey Bell to the Williams seat. Bell, who lost to Sen. Bill Bradley (D) in the 1978 Senate race, admitted this week that he is trailing Mrs. Fenwick by 2 to 1 in his own polls.

Both Mrs. Fenwick and Bell praised Brady, declaring that he would make an excellent choice.

Brady and Mrs. Fenwick both live near Far Hills, one of the wealthiest residential areas of the state. They are long-time family friends.

Williams, convicted a year ago in the Abscam scandal, resigned last month to avoid a Senate vote that almost certainly would have expelled him. Although Kean was aware for months that Williams was certain to be forced out of the Senate, he waited until Williams' resignation and then consulted with at least 100 Republican leaders throughout the state.

By selecting a caretaker, Kean would avoid splitting the conservative (Bell) and moderate (Mrs. Fenwick) wings of the state Republican party.

Kean, long considered a moderate Republican, embraced President Reagan and much of his supply side rhetoric in his election campaign last year.

Brady is a fourth generation Irish-American. His great-grandfather, Anthony Nicholas Brady, came to New Jersey from Ireland during the famines of the 1880s.

But he became wealthy thanks to his involvement in some early electrical inventions. He helped form several small electrical companies in Albany and Brooklyn that eventually became Consolidated Edison.

His son, James Cox Brady, branched out into auto manufacturing and horse racing--the Hamilton Farms and the Maxwell Company--which produced the classic automobile, the Maxwell.

Chrysler Corp. bought out the Maxwell firm but Brady helped back a group of Chrysler engineers who formed the Purolator Corp.

The next son, James Cox Brady Jr., went into investment banking. After graduating from Yale University and Harvard Law School, the current Brady joined the Dillon Read investment firm.

By 1961, Brady was vice president of the firm. In 1970 he was credited with reorganizing and revitalizing the company by expanding its operations.