The Senate Finance Committee completed most action yesterday on a tax cut for next year of $39 billion, far in excess of its original target.

In a fit of election-year largesse, the committee voted in yesterday's session:

To cut the capital gains tax, so that 70 percent of most such gains would be exempt from taxes instead of 50 percent.

To increase to $1,750 for individuals and $2,000 for married couples the income that can be set aside tax-free in a so-called IRA, or individual retirement account, and for the first time to let persons, covered by pension plans set aside some money in this way.

To cut the corporate income Tax rate to 44 percent from 46.

To phase in, over the next two years, other cuts for small businesses.

Amid much laughter, the Senate panel voted to tack its tax measure onto a minor House-passed bill that would let six bells for the United Methodist Church of Washington enter the country duty-free. The Constitution says tax legislation must originate in the House, so the committee's tax bill must be sent to the Senate floor attached to a House tax bill.

Despite the speed and unity with which the Finance Committee wrote its bill this week, much doubt remains about whether a tax cut will be enacted this year.

The House has so far been unenthusiastic, and yesterday the Senate Budget Committee refused to leave room in its fiscal 1981 budget resolution for a 1981 tax cut, casting doubt on support within the Senate at large for the bill.

Al Ullman (D-Ore.), chairman of the influential House Ways and Means Committee and an opponent of a tax bill for this year, said yesterday that at mostly at "closely contained" bill could be passed this year, and that the House would be better able to "appraise the status of the economy and of the Senate action" after the Labor Day recess.

The Senate committee capital gains tax change would mainly benefit higher-income groups.

Other personal tax cuts voted Wednesday -- which will cost the Treasury about $22 billion next year and $42 billion annually by 1985 -- are intended roughly to compensate each income group for the extra income tax that will be due net year because of inflation, and for scheduled increases in the Social Security tax.

The average personal tax cut from the bill was put in the neighborhood of $250, not counting the capital gains and savings changes.

But for most people, the tax cuts under the bill would barely offset the increases due next year. Some would actually end up worse off.

Final details on the cuts in tax rates are still to be worked out, and the Senate committee will settle on them this morning. It voted 19 to 0 yesterday in favor of the overall package.

Committee Chairman Russell Long (d-La.), who steered the bill swiftly and effectively through the committee, was anxious yesterday to rebut the suggestion that Senate Democrats had "panicked" when they voted for presentation of a tax bill by Sept 3. "We didn't panic," he said. "We just took a reading on where the votes were, and saw they weren't there" to defeat a Republican amendment favoring an early tax cut.

He said the evidence for a tax cut was "compelling and overwhelming" and the committee had put on a "good display of bipartisanship."

Other senators joined in the self-congratulation.

John C. Danforth (R-Mo.) pointed out that while six members of the committee including the chairman are running for reelection this year, 14 are not.

Lloyd M. Bentsen (D-Tex.), who has supported a tax cut all along, said the committee is "trying to fight inflation with production lines, not with unemployment lines."

The proposals approved yesterday would add about $6 billion or $7 billion in calendar 1981 cuts to those voted earlier in the week. The full-year cost of the package by 1985 is now estimated at $75 billion, up from the $70 billion informal target the committee set itself Monday.

But for fiscal 1981, the cost of the total bill would be less than $20 billion, the committee staff estimates. This is because the fiscal year starts Oct. 1, 1980, and most of the cuts would not be effective until next Jan. 1, and because some tax payments are delayed. It is the fiscal year cost which matters for the federal budget.

The capital gains tax cut could bring the maximum tax rate payable on capital gains down to 20.1 percent from its present 28 percent. Businesses would be able to opt for a tax of 20 percent on their capital gains rather than the previous optional rate of 28 percent. These changes are estimated to cost about $800 million in fiscal 1981, rising to more than $2.5 bilion for the calendar year and to $3.5 billion by 1985.

The corporate tax rate reduction would cost the Treasury $700 million in fiscal 1981, $1.5 billion in calendar 1981, and $5 billion a year by 1985.

Under a proposal by Sen. Gaylord Nelson (D-Wisc.), small businesses would benefit next year from an increase from $100,000 to $150,000 in the point beyond which the full corporate tax rate is payable, as well as a cut in the special tax rate on businesses earning less than $25,000 a year.

Further changes for 1982, proposed by Sen. John H. Chafee (R-R.I.), would cut taxes paid by firms making between $50,000 and $200,000. The total Treasury cost of this and other measures to help small business is put by the committee staff at about $1 billion next year, rising to more than $1.5 billion in 1985.

Other measures agreed to by the committee yesterday include:

Bigger tax break for Americans working abroad, slanted to help most those outside Europe and Canada. Estimated cost: about $500 million eventually, a litle less in 1981.

A new "Limited Employe Retirement Account" under which those in pension plans could defer tax on savings of up to $1,000 a year for retirement. pThis, with the increased individual Retirement Account, would cost about $1 billion in calendar 1981 and $1.6 billion by 1985.

A change in the employe stock option plan aimed at helping firms with big payroll rather than capital-intensive business. Estimated cost: $300 million in 1981, rising to $4.3 billion in 1985 (although the committee voted it only until 1983).

More generous tax treatment of research and development expenditures by business. Estimated cost: less than $500 million in 1981, rising only slightly by 1985.