Japan has told the United States it is planning to unveil a massive new stimulus package later this month that will spur the lagging Japanese economy to a growth rate of between 7 and 7 1/2 per cent - up from the 5 1/2 to 6 per cent expected for this year.

American sources, who spoke guardedly of the Japanese proposal yesterday, declined to disclose either the actual size or the composition of the Japanese package.

The proposal is expected to meet with general approval of the United States, which has been pressing the Japanese to bolster their economy with added stimulus as a way to bring an increase in purchases of import and hoped for a reduction of Tokyo's burgeoning current account surplus.

Although the United States still is pressing for additional steps to deal with the surplus problem, the stimulus package is expected to be the heart of the Japanese effort next year. Tokyo has insisted it wants to keep the size of the program secret until next week's unveiling.

The disclosures came as Japan made public yesterday a list of more than 300 proposed tariff cuts and import quota liberalizations it has outlined in talks with American officials this week - most of them relatively small and not involving so-called "big-ticket" items.

Although Tokyo estimated the reductions would affect some $2.2 billion worth of worldwide exports to Japan, only about $735 million of U.S. goods would be involved, and not nearly all the American negotiators have been secking clearance to export.

The major concussions to the United States were a small increase in the amount of American hotel-grade beef and oranges allowed into Japan and small cuts in tariffs for computers, airplanes and cars. U.S. silk, leather or consumer goods were not mentioned.

U.S. officials indicated both the scope and the size of the cuts was insufficient to make much difference in American export levels to Japan. Alan W. Wolff, the deputy US. special trade representative, labeled the tariff cuts "rather modest."

The action came as Japan's newly appointed trade representative, Nobuhiko Ushiba, continued talks here with top government officials and members of Congress, apparently not retreating from his position that the tariff cuts were all Japan can do.

Ushiba is scheduled to meet briefly with President Carter this afternoon and then fly to Brussels for a surprise conference with officials of the European Common Market, which is seeking similar concessions by the Japanese.

The disclosure of the tariff-cut proposals apparently was designed to bolster Tokyo's assertions that the list was a substantial one. The Fukuda government has pledged to make the changes on its own initiative April 1 in advance of worldwide trade talks in 1978.

Top U.S. officials declined to discuss reports concerning the expected Japanese stimulus package. Robert S. Strauss, President Carter's special trade representative, and the size and makeup of any new Japanese economic measures "are up to the Japanese to decide."