The Carter administration is considering a thorough overhaul of the nation's Social Security system in 1979, involving not only the financing of the present program, but changes in its scope and benefits as well.

With White House approval, a top level interagency task force is reviewing the present program with a view toward drafting new proposals to be included in President Carter's budget and economic message in January.

If policymakers go ahead with all the options, the plan could call for everything from requiring federal workers to participate to financing all but the old-age retirement program with general income-tax revenues.

At the same time, the administration also is studying new tax breaks to spur capital investment, from faster depreciation writeoffs for business to new export incentives and an expansion of the investment tax credit.

Officials say the administration does not plan to propose another big general tax reduction next year - either to stimulate the economy or protect workers from inflation - or to revive its ill-fated "tax-reform" program.

Strategists stress that planning for the Social Security proposals still is in its early stages, and the final decisions have been made yet. Carter is expected to seek major changes, but it's not clear how far he will go.

However, officials say any plan almost certainly would be more sweeping than the package the White House proposed in 1977 for using some general revenues to finance the disability insurance portion of the present program.

Congress rejected the administration's proposals in 1977 and chose instead to raise Social Security taxes, with a large increase scheduled for next January. Planners are convinced there will be an outcry for further overhaul when the increased tax rates go into effect.

The task force consisting of officials from the Departments a Treasury and Health, Education and Welfare, is expected to consider these elements:

Should the government begin using general income-tax revenues to help finance portions of the present program - at least the so-called "welfare" provisions not directly affecting the old-age retirement plan.

EssentiallY, that would amount to stripping the Social Security program of all provisions not related to the old portions to the regular budget. Congressional squabbles have barred this before.

Should Congress mandate broader coverage for the program, to help keep it afloat financially? Primarily, that means requiring federal workers, who now are exempt from Social Security, to join in with private employees.

Should Congress trim back the growth in some benefits - and tax levels - by easing requirements that both grow in line with inflation. All three measures have been recommended from time to time by earlier study groups.