The General Accounting Office, which will have the final word on $11.7 billion of budget cuts suddenly demanded by the new balanced-budget act, will have just five days, including a weekend, to deliver its verdict to President Reagan.

"We'll just have to drop other work, but the problem isn't going to be finding enough bodies. It's going to be finding enough people with the required skills," the GAO's general counsel, Harry R. Van Cleve, said last week.

Under the new legislation, already being challenged in court, the White House Office of Management and Budget and the Congressional Budget Office will do the initial work of calculating program-by-program spending reductions under a complicated formula spelled out in the bill.

Their report, as one congressional summary puts it, "would contain all of the information needed to prepare the presidential order" for Reagan's signature "and, in essence, would constitute a draft order."

The draft will be sent to the GAO Jan. 15, and the head of that agency, Comptroller General Charles A. Bowsher, will be required to have it reviewed, make any necessary changes and submit it to the White House on Jan. 20, a Monday.

Under the law, the president is obligated to implement cuts set out in Bowsher's report. GAO officials said they are ruefully envisioning headlines such as "Comptroller General Cuts School Lunches."

Such critics of the new law as Alan B. Morrison, head of the Public Citizen Litigation Group, maintain that the GAO is simply meant to give a quick blessing to the cuts and cast "an attempted constitutional gloss" over the procedure.

Under a 1976 Supreme Court decision, any appointed officials exercising "significant authority pursuant to the laws of the United States" must be appointed by the president. While the GAO is generally considered a congressional watchdog on the public purse, the comptroller general, unlike the director of the Congressional Budget Office, is a presidential appointee.

In other words, Morrison charges, the GAO "is supposed to wave its magic wand and constitutionalize an otherwise unconstitutional report. They've never done a thing with the budget. Now they have three working days to review an extraordinarily complicated report with people who have no expertise in the field."

Van Cleve disagreed with the notion that the GAO's role will be perfunctory. He said it will depend in large part on how much work the OMB and the CBO do before submitting their findings.

"If they do all the mathematical work, then that's what we'll review," he said. "But if they don't do it, then we will.

"We've got to arrive at a percentage of reduction needed to bring spending down by $11.7 billion for fiscal 1986, the year we're in now, and then go through a fairly complicated procedure to determine against what appropriation accounts the percentage is to be applied. There's a three- or four-page list in the law of untouchable and limited untouchable programs," he said.

There are other complexities. "In the crop-support program," Van Cleve said, "there are agreements that are made in 1986 that take effect in 1987 because that's the harvest year. Now we apparently have to reduce the 1986 level of agreements. But what that has to do with 1986 expenditures, I don't understand."

Under the new law, if the OMB and the CBO disagree, their directors are to "average their findings so as to report a single consistent set of findings, along with each director's own finding," to the comptroller general.

The GAO is supposed to make changes only where necessary, having a "due regard" for the OMB-CBO draft and making sure to "explain fully" any revisions.

It will not be easy, Van Cleve acknowledged. "The first year is on a very short fuse," he said. "We don't have any systems worked out yet, and we have no relationships established with the OMB and the CBO . . . . We've just begun to talk about what we need to do."