U.S. Ambassador to Czechoslovakia Jack F. Matlock Jr. will be named the Reagan administration's director of Soviet and East bloc affairs, administration officials said last night.

Matlock, 53, will replace Richard Pipes, who resigned late last year. Like Pipes, Matlock is considered a tough-minded conservative who has militant views about the Soviet Union.

He will report to national security adviser William P. Clark, who chose him for the position after consulting with Secretary of State George P. Shultz, administration officials said. Clark is known to consider Matlock the "finest Sovietologist in the State Department," according to one knowledgeable administration source.

Pipes, who was considered the principal developer of administration policy options for the Soviet Union, resigned in December to return to Harvard but has remained a consultant on Soviet affairs to the National Security Council. He also is known to think highly of Matlock, who shares many of his views about the Soviet Union.

It was Pipes who articulated the belief, subsequently accepted by President Reagan, that the Soviet Union faces severe economic stresses from within and may be susceptible to pressure from the United States and its allies.

Clark became aware of Matlock's abilities when Clark served at the State Department. Besides Czechoslovakia, Matlock has experience in Poland and as a deputy chief of mission at the U.S. Embassy in Moscow, and is considered highly informed about the relationship between the Soviet Union and its Communist allies in eastern Europe.

His appointment is part of a move by Clark to strengthen the Soviet affairs section of the National Security Council. Two other officials who have served in the interim since Pipes' resignation also will remain on the job, officials said.

Reagan is known to feel that U.S.-Soviet relations are entering a critical phase, and the efforts of the National Security Council are likely to be increasingly focused on U.S.-Soviet affairs.

Matlock, who consulted with White House and State Department officials in Washington last week, will report to his new post in about 10 days, officials said, but may not resign his ambassadorship immediately. They added that when he resigns officially, the vacancy in Czechoslovakia probably will be filled by a professional U.S. diplomat.