For almost a year, legislators have debated and approved far-reaching changes in the tax code knowing that their work could be altered when the House and Senate worked out the final blueprint for tax overhaul.

That era ends today.

Twenty-two members of the House and Seante, the final conferees on the tax package, will begin today the final rewrite on a host of tax-code provisions that, if signed by the president, will affect every U.S. taxpayer.

The cast of characters was made complete yesterday, as House Ways and Means Committee Chairman Dan Rostenkowski (D-Ill.) named six Democratic allies to join him and four committee Republicans as the House contingent to the conference. They will meet with Senate Finance Committee Chairman Bob Packwood (R-Ore.) and his five other GOP and five Democratic conferees.

The House Democratic conferees are: Reps. Rostenkowski, J.J. (Jake) Pickle (Tex.), Charles B. Rangel (N.Y.), Fortney H. (Pete) Stark (Calif.), Richard A. Gephardt (Mo.), Marty Russo (Ill.) and Donald J. Pease (D-Ohio). The Republican conferees are Reps. John J. Duncan (Tenn.), Bill Archer (Tex.) Guy Vander Jagt (Mich.) and Philip M. Crane (Ill.).

The importance of the conference was shown by the fact that both Rostenkowski and Packwood broke with tradition and selected conferees based on loyalty and ideology as well as seniority. Rostenkowski bypassed nine committee Democrats to select Pease, for example.

"He wants to control the conference, and I would tip the balance on energy," said a disappointed Rep. James R. Jones (D-Okla.), the sixth-ranking Democrat on the committee, who was edged out of the conference.

Before naming the conferees yesterday, the House approved, 338 to 61, a nonbinding resolution asking the conferees to agree to the Senate bill's low rates and doubled personal exemption, to $2,000, but again endorsing the House's more-favorable treatment of the tax-deferred Individual Retirement Accounts, at least for low- and middle-income taxpayers.

Despite the importance of the conferees and the conference, many of the provisions in the House and Senate bills are identical, and thus virtually guaranteed of becoming law.

The deduction for two-earner families is gone, as is income averaging. Federal workers will start paying taxes on a portion of their pension immediately on retirement. Low-income Americans will see vast reductions in their taxes, and about 6 million of them will no longer have to pay federal income taxes.

Only 80 percent of the cost of business meals and entertainment will be deductible. Companies will no longer have the investment tax credit to subsidize up to 10 percent of the cost of investment in equipment and machinery. Tax credits for research and for rehabilitation of historic buildings will remain, in modified form.

The general framework of the rest of the bill has been endorsed in principle by Rostenkowski and Packwood. They both like the low rates of the Senate bill, which would cut the top tax rate for individuals almost in half, from 50 percent to 27 percent. (The top rate in the House bill is 38 percent.) And they both hope the final plan will cut the taxes of middle-income taxpayers by the 9 percent to 10 percent of the House measure rather than the 5 percent to 8 percent in the Senate version.

Still unsettled in this bargain, however, is the tough question of where the tax revenue will be raised so that the tax package can provide low rates and big individual tax cuts without increasing the federal deficit.

"When you get down to the nitty-grity of how to pay for it, it will not be easy at all," said Sen. Lloyd Bentsen (D-Tex.). "Obviously, some of it will come from business."

Already, the Senate bill would raise corporate taxes by $100 billion over five years, granting a tax cut of equal size to individual taxpayers. The House bill officially would raise business taxes by $140 billion over five years, although revised estimates could bring that increase to as much as $160 billion. Because the conferees will debate individual provisions before business provisions, they are likely to turn to latter to pay for the former.

Packwood has said the final bill could include in an additional $20 billion to $25 billion on business taxes on top of the Senate bill's increase. The conferees must decide who will bear that burden. One possiblity is the Senate's plan for depreciation, the system by which companies account for the loss in value of aging equipment and real estate, which is more generous than the House version and than current law.

The Senate also is kinder to timber producers and oil and gas producers, and Packwood and ranking Finance Democrat Russell B. Long (D-La.) can be expected to fight hard for those home-state provisions. A Senate crackdown on tax shelters not included in the House bill, on the other hand, is likely to be accepted in modified form.

The conference also could choose to be tougher on the companies that fare well under both the House and Senate bills, principally retailing and other service industries.

Those industries now pay a high tax rate and would do well under the corporate rate reductions in the bills, from 46 percent to 33 percent in the Senate measure and to 36 percent in the House bill. A higher corporate rate would affect them disproportionately.

"There's going to be disappointment for the business interests who thought they were being invited to a banquet and find they're being served cold hot dogs," said Wayne Thevenot of the National Realty Committee.