The House Ways and Means Committee said yesterday it will aim for a tax cut of as much as $50.4 billion for fiscal 1982, and $8.8 billion in cuts in the big spending programs under its jurisdiction, thus broadly concurring in the Reagan administration's budget goals.

But the committee indicated serious doubts about the president's economic assumptions, and cautioned that it would approve a smaller tax cut, if necessary, to hold the federal budget deficit at the $45 billion level projected by the administration. Thus the Ways and Means members said in their report to the House Budget Committee, setting out their intentions for the coming year, that their "position is necessarily a tentative one at this time."

It is also virtually certain that the committee will change the president's tax proposals substantially even if it approves a cut of about the same size, sources said. The panel allowed for $1 billion less in net tax cuts for 1982 than the administration proposes, partly because it assumes that taxes on unemployment benefits will be increased as it has suggested.

Ranking Republican Barber Conable (N.Y.) hailed the swift approval of figures close to those of the administration as a "key step in moving the [administration's] tax and saving proposals along toward enactment" while acknowledging that there would be "difficulties from time to time" in winning approval for the president's specific recommendations.

Earlier yesterday budget director David Stockman told the Senate Finance Committee that the administration favors extending the present exclusion from taxes of some interest and dividend income, although there was no provision made for this in the Reagan budget numbers published last week. This year and next year the first $200 of such income for a single person, or $400 for a married couple, is excluded from tax, but this relief is due to end in 1983. Stockman said the administration would deal with this "when the time comes." By 1984 this change would cost $3.3 billion, congressional estimates show.

Meanwhile, the Congressional Black Caucus yesterday proposed a radically different alternative to the Reagan budget. Saying that it would support "justified reductions in federal spending, but not on the back of the poor and disadvantaged," the caucus proposed a much smaller tax cut than Reagan's, aimed at helping the poor and middle income groups, a smaller increase than Reagan's in defense spending, and continued spending on social programs and help for cities in poorer areas.

This would achieve some of the same goals set by Reagan, such as higher employment in the private sector, a lower federal deficit and less government waste, the caucus said.

N a separate hearing yesterday, the chairman of the Council of Economic Advisers, Murray Weidenbaum, urged Congress to withhold approval of a constitutional amendment to balance the budget and give the Reagan program a chance to work first.

Although preliminary votes this week on the budget numbers for 1982 have allowed for tax cuts and some key spending cuts of the size requested by the president, the distribution of both may well differ from the administration's proposals. Senate Finance Committee Chairman Bob Dole (R-Kan.) said pointedly yesterday. "I told Pete Domenici I am not going to make the Finance Committee a subcommittee of the Budget Committee" (which the New Mexico Republican chairs) and just cut programs where the Budget Committee says.