Less than 24 hours after President Reagan outlined plans to cut the federal food stamp program sharply, Agriculture Secretary John R. Block appealed "urgently" to Congress yesterday for money to keep the program going.

Block appearing before the House Appropriations subcommittee on agriculture, said the administration will need about $1.2 billion to keep food-stamp benefits at current levels through this fiscal year.

Appearances were deceiving, however. In his first meeting with the powerful subcommittee -- a sort of love-feast, in fact -- Block made it clear that there's going to be pain and scrimping in fiscal 1982, beginning Cot. 1.

But for this year, "Without additional funds, I will have no choice but to reduce benefits to those in need of food stamps," the secretary said. "Your assistance in obtaining passage of these supplemental appropriations is urgently needed and will be greatly appreciated."

There was every sign that the subcommittee will oblige Block and, judging by the tone of yesterday's questions, will be giving him more help than he wants.

Several members pointedly challenged the Reagan administration's proposals to scale back the food stamp program by about $1.8 billion next year.

Reps. Bob Traxler (D-Mich.), Daniel Akaka (D-Hawaii), Matthew F. McHugh (D-N.Y.) and Jerry Lewis (R-Calif.) expressed concern about the plan to reduce family stamp allotments if a child gets free school lunches.

Traxler and McHugh pressed Block with the theory that a school-lunch offset on stamps would deprive some of the neediest families of food assistance and create more administrtive red tape.

"The average per-meal subsidy for a food-stamp recipient is between 38 and 44 cents," McHugh said. "That subsidy is rather low -- and so is the gross income of the average family on food stamps. The children on the lunch program and food stamps are not being overfed."

Block, however, insisted that the lunch-and-stamps combination is a "duplication" that needs to be eliminated.

While he caught some flak on the food issue, Block generally got an unabashedly warm welcome from the subcommittee, which, in its way, wields as much or more clout on agricultural policy as the secretary himself.

As Chairman Jamie L. Whitten (D-Miss.) noted dryly, Congress for 38 straight years has stood behind the soil-conservation program he has pushed past the oppostion of successive administrations in the White House.

And another veteran, Rep. William H. Natcher (D-Ky.), in a monologue on the fiscal contributions of tobacco to the American economy, said he hoped Block was paying close attention to his wish that more tobacco research money go into the Reagan budget.

If block doesn't do it, Natcher indicated, he will see that tobacco is taken care of in fiscal 1982. Block moved his head positively, signaling that he got the gist of the lecture.

The secretary said details of the 1982 departmental budget won't be available until March 10, but he said that as much as $1 billion more in unspecified USDA budget cuts -- beyond those already announced -- are likely to be proposed.