The chairman of the House Post Office and Civil Service Committee moved yesterday to reassure federal employes that they will not lose Civil Service pensions if Congress forces Social Security coverage on new federal workers as part of a Social Security rescue package.

Meanwhile, the National Association of Manufacturers' 214-member board of directors unanimously endorsed the Social Security rescue plan, which was recommended Jan. 15 by a special presidential commission and supported by President Reagan and House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.).

Another group, the National Council on the Aging, said that the rescue package made "significant progress" toward saving Social Security, but that it could not support one part of it, a six-month delay in the cost-of-living adjustment.

The assurances on Civil Service pensions, which could help defuse opposition to the Social Security plan by federal employe unions, came from Rep. William D. Ford (D-Mich.) in a letter to House Ways and Means Committee Chairman Dan Rostenskowski (D-Ill.).

Ford said that if Congress enacts some form of Social Security coverage for new federal employes his committee would "act expeditiously and responsibly to guarantee that federal employes covered by Social Security also will earn retirement benefits under the Civil Service retirement system, and that benefits for current employes under the CSRS will be maintained."

Sources said that the letter indicated Ford's commitment to pass legislation to make sure that future employes would get both a Social Security pension and a new supplemental federal pension, and that current Civil Service employes would not see their benefits wiped out or the financing undercut.

The rescue plan recommends that, starting in 1984, all newly hired federal employes be covered by Social Security, leaving those already on the federal payroll in the existing Civil Service retirement system. Rostenkowski, in a letter to Ford earlier this week, said he expects both the Ways and Means Committee and the full House to approve that provision.

Federal worker unions have been waging a campaign against the proposal, charging that it would endanger financing for their system and leave new employes with only a Social Security pension.