William Brock, President-elect Ronald Reagan's choice for U.S. Trade Representative, said yesterday that the Japanese should be prepared to "play some hard ball" with the United States on the trade field.

But Brock, at bat during his Senate confirmation hearing yesterday, was thrown nothing but softball questions from Senate Finance Committee members, who quickly approved Brock's nomination adn sent it to the Senate for confirmation.

Brock, a former U.S. Senator from Tennessee and a former member of the Senate Finance Committee, said the United States has "got to be, frankly, fairly tough" in trading with the Japanese. Japan believes the U.S. government doesn't care enough about trade to challenge them, but "by golly, we do."

As for American businessmen, "It's time some of them got off their duffs" and export more, Brock said.

"As the Japanese have adopted our national pasttime, I am sure they will understand when I express the belief that we are ready to take our turn at bat, and we are not at all reluctant to play some good hard ball," Brock said. b

Several senators also warned Brock not to be intimidated by other Cabinet-level officials who may feel they are more important than he is or attempt to usurp some of his trade authority. "Tell those people you're the same rank they are," said Sen. Russell B. Long (D-La.).

Brock said that he and Commerce Secretary-designate Malcolm Baldridge have discussed their roles and that they plan to cooperate with each other. In addition, chairmanship of the administration's trade policy committee by law goes to the U.S. trade representative, Brock added.

The most pressing trade matters he will address as trade representative, Brock said, would be help for the automobile industry and renewal of the multifiber arrangement to aid the American textile industry and improve trading relations with the Japanese.

Brock also said he will watch the implications of enlargement of the Common Market, which soon will grow from 10 to 12 nations and stress agricultural exports. He said he is considering creating a high-ranking agricultural trade position.