House and Senate conferees agreed yesterday to end payment of the minimum Social Security benefit next March, even as a House Ways and Means subcommittee was laying plans for legislation to keep the benefit for the 3 million people who receive it.

Action on the minimum benefit, which reflects votes taken earlier by both houses to kill it, came as more than 250 conferees on nearly $40 billion worth of spending cuts for next year struggled to meet this week's target date for completing their work.

Rep. Leon E. Panetta (D-Calif.), who is monitoring the 58 "mini-conferences" for the House Budget Committee, told reporters that about 35 have substantially completed their work and that the cumulative savings are "very much on target . . . give or take a billion."

Among other major decisions yesterday, the conferees:

Added nearly $200 million in cuts to reductions already approved for the food stamp program -- for a total of $1.65 billion in food stamp cutbacks. The additional cuts, which were demanded by Senate Agriculture Committee Chairman Jesse Helms (R-N.C.), reduce benefit levels, especially for recipients who hold jobs. In all, 1 million recipients will be dropped from the rolls and another 1 million will receive reduced benefits.

Agreed to provide $950 million to continue the Head Start program for pre-schoolers that the House had inadvertently dropped from its version of the legislation and $715 million for programs under the Older Americans Act.

Created a separate $354 million community services block grant, instead of combining community action programs into a social services block grant, as the administration had wanted.

Raised the initial hospital charge a Medicare patient must pay before government compensation starts, from $228 to $256 next year, rising to $323 in 1984. But a House proposal to make Medicare patients pay a special $1-a-day fee for the first 60 days of a hospital stay was dropped. Medicare reimbursement for nursing costs was limited to 105 percent of normal nursing costs, instead of the current 108.5 percent, a move strongly opposed by hospitals.

Picked up additional savings from Social security by delaying payments for new retirees for what amounts to a month, saving $190 million, and by stopping parent's benefits when their youngest child reaches 16 instead of 18, which would save $496 million by the time the new provision is fully effective in 1984.

Still to be resolved was the administration's proposed "cap" on growth of Medicaid.

All that is at issue in the Social Security conference is when the minimum benefit would end. The Senate had proposed an immediate end, while the House proposed continued payments until next April. Under the compromise, next March's check would be the last for current recipients. New retirees would get the minimum benefit only through this December.

Dthe minimum benefit of $122 a month, or more in some cases, goes to people who did not contribute enough to Social Security during their working careers to add up to that much in monthly benefits. Because other forms of government compensation will not make up the difference for thousands of elderly people, Congress is under heavy pressure to retreat from its earlier decision to repeal the benefit. The move to repeal it was aimed at saving $1 billion next year and $7 billon over the next five years.

The delay in termination of benefits will serve two purposes: Congress will have time to change its mind, and the Social Security Administration will have time to identify recipients and help them shift to other forms of compensation, where available.

"This is going to affect the life styles of a great many people," said Ways and Means Committee Chairman Dan Rostenkowski (D-Ill.). "We're going to have to give these people due time in order to apply for SSI (NEW-LINE)(supplemental security income and make other adjustments.

As for a possible about-face by Congress, Rep. J. J. Pickle (D-Tex.) said yesterday that his Ways and Means subcommittee on Social Security will reaffirm its earlier decision to protect current retirees and kill the benefit only for new retirees.

Asserting that "there are too many cross-currents now," Pickle said he will probably not bring up the minimum benefit when the subcommittee meets today to resume work on legislation designed to solvef the Social Security system's financial problems. But "it's one of the first things we're going to get to," he added.

Pickle's hand was strengthened by a 405-to-13 vote of the House last Tuesday to prevent any Social Security cutbacks for those already on the rolls, even though the Senate refused to go along when confronted by a similar proposal as a rider to the administration's tax bill.

Even though Congress may change its mind and protect current retirees, conferees were taking no chances. They decided that notices should be sent out in December warning of the projected March cutoff. CAPTION: Picture, Among the more than 250 conferees working on about $40 billion in spending cuts are Sen. Russell B. Long (D-La.) Sens. John Heinz (R-Pa.), and John C. Danforth (R-Mo.), and Reps. Rostenkowski and Sam M. Gibbons (D-Fla.). By James K. W. Atherton -- The Washington Post