Calling it a "central element in our African policy," President Carter yesterday signed a bill restoring the embargo on U.S. use of Rhodesian chrome.

The President also proposed a $7.4 billion foreign aid package for fiscal 1978, with a slight reduction in military aid, and more money for economic assistance, especially for developing nations.

Former President Ford had proposed a $6.3 billion foreign aid program.

Carter had called during his campaign for the U.S. to stop importing Rhodesian chrome, which it has been doing since 1971 under an exemption to a United Nations ban. The exemption was sponsored by Sen. Harry F. Byrd Jr., (Ind. Va.).

The President said yesterday that "as a matter of equity," he ordered an exemption from the ban for shipments "now in transit." The new embargo puts us on the side of what's right and proper," he said. And he called the move "a positive step" toward peaceful establishment of black majority rule in Rhodesia.

Carter's foreign aid package provides:

$2.8 billion for aid to developing nations through such groups as the World Bank, an increase of about 30 per cent.

$1.4 billion for direct U.S. loans or grants to other countries, about the same as Ford asked. Some $50 million is included as the first major U.S. contribution to a long-term drought recovery program in the Sahel region of northern Africa.

$923 million for Food For Peace, the same as Ford requested.

$1.8 billion for nonmilitary economic assistance to "countries in whose stability the United States has a very keen interest." That category is up $428 million, with most of the increase going to Middle East nations, and Israel claiming the largest share.

Some $600 million kept on hand for world lending institutions like the World Bank, in case borrowing nations default on their loans. There have never been such defaults, officials said, and similar guarantees by previous administrations have never been spent.

The military assistance program outlined by Carter was for $663 million, down by $3 million from the Ford request.

In other White House items, Carter confirmed that he will nominate Frank Press, a geologist, as White House science adviser. Press is a department chairman at the Massachussetts Institute of Technology.

The President also said he will nominate Joan Claybrook of Baltimore to be administrator of the National Highway Traffic Safety Administration; William J. Beckham Jr., deputy mayor of Detroit, to be assistant treasury secretary for administration; Roger C. Altman, a New York City investment banker, to be an assistant treasury secretary, and M. Rupert Cutler of Michigan State University to be an assistant agriculture secretary.