House-Senate conferees bowed yesterday to White House veto threats as they scaled back their spending plans for the rest of the 1983 fiscal year and approved a critical supplemental appropriations bill that is needed to continue funding food stamps after Aug. 1.

The bill includes $7 billion exclusive of $8.5 billion for the International Monetary Fund, which the Senate insisted on and the House resisted to the end, meaning the bill could cost as much as $15.5 billion if the IMF money is eventually approved.

The IMF issue will have to be resolved by the two chambers when they vote on the conferees' report next week, presumably after the House decides whether to authorize the additional U.S. contribution to the fund.

Although there was no immediate word from the White House whether the bill was acceptable, Senate Appropriations Committee Chairman Mark O. Hatfield (R-Ore.) indicated repeatedly that it had a good chance of being signed if the conferees accepted the cuts that were eventually approved. "We've done all they asked us to do," said a Senate staff aide after the conferees lopped off $370 million for strategic petroleum reserve storage facilities, $225 million for health insurance for the unemployed and assorted other amounts in order to get the measure within reach of the limits demanded by President Reagan.

With the conference hung up to the end over the petroleum reserve storage facilities, several Democrats contended the money should be approved even if it means a veto. But Hatfield, warning the conferees they were "looking a veto square in the face," insisted on accommodations to gain Reagan's signature and eventually prevailed.

Even though the outcome appeared to signal an important early victory for his strategy of forcing domestic spending cuts by threatening vetoes, Reagan was not a runaway winner.

The conferees not only left hanging the IMF money, which the president wanted, but they also cut in half his $50 million request for new military aid to El Salvador.

On the bill as a whole, an Office of Management and Budget spokesman said OMB could not yet take a position but added that the conferees had made "a great deal of progress in bringing the bill down to an acceptable range."

One reason Hatfield was able to prevail in scaling back the bill to meet Reagan's demands was the fact that it contains $1.2 billion to continue funding food stamps after the end of this month, when existing funds will be in danger of running out.

The Agriculture Department has warned states that about 22 million food stamp recipients face a 25 percent cut in benefits unless the bill is passed and signed into law by Aug. 1.

Although the conferees dropped $225 million for health insurance for the unemployed, the conferees indicated an appropriation for the insurance would be provided after Congress agrees on what kind of program it wants.

Similarly, the conferees agreed that a delay in funding for the strategic petroleum storage facilities should not jeopardize the 220,000 barrel-per-day congressional target for filling the reserve.

In the midst of arguing over spending cuts, the conferees themselves plunged into a testy dispute after senators began earmarking funds for specific colleges and universities in their home states.

Rep. William H. Natcher (D-Ky.) protested that he had fended off similar requests from House members, only to find that the senators were doing the same.

They included $20.4 million for a biomedical facility at the University of Oregon in Hatfield's home state, as well as smaller sums for facilities in the home states of Senate Appropriations Committee members Warren B. Rudman (R-N.H.), Lowell P. Weicker Jr. (R-Conn.) and Daniel K. Inouye (D-Hawaii).

Hatfield responded that he had intended to keep the bill free of such pork-barrel designations until he got a call from House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) requesting $7.5 million for a library at Boston College.

"I said I was first in line" if the conferees were going to play the earmarking game, Hatfield told his colleagues. O'Neill, he added, said that was fine with him.

With the support of his House colleagues, Natcher forced the senators to drop the university designations from the bill, although an accompanying report will suggest to the administration where Congress wants the money to go.

Senators were not the only losers. "Do you want me to start crying? . . . . I got nothing," moaned Rep. Silvio O. Conte (R-Mass.) after Boston College got caught in the purge.