The State Department yesterday strongly suggested that the administration plans to go ahead with a large-scale arms sale to Saudi Arabia despite the opposition of 64 senators, and officials said a final decision on what arms will be sold is likely to be made next week.

State Department spokeswoman Phyllis E. Oakley said the $1.4 billion arms sale under consideration would be in America's interest and that it is "unfortunate" that the sale is being opposed on Capitol Hill "when the Saudis are providing critical support to U.S. naval operations in the {Persian} Gulf in ways which meet our mutual interests and needs, and in ways which many in Congress have long urged."

Oakley declined to make public details of the support that Saudi Arabia is providing, but she said the information has been provided to members of Congress in classified reports.

At least 217 House members are adding their opposition to that of the senators, which was expressed in a letter presented to the White House on Friday. Rep. Larry Smith (D-Fla.) and other members of the House Foreign Affairs Committee are scheduled to make public today the House members' letter to President Reagan opposing the sale.

Congressional sources said national security affairs adviser Frank C. Carlucci was at the Capitol yesterday to discuss the arms sale with key lawmakers and that senior State Department officials have spent much time in similar consultations.

No decision on the details of the arms package is likely before the end of the Jewish high holy days this weekend, according to an administration official. He added that the arms proposal probably will have to be submitted to Congress next week in order to provide the required 50-day opportunity for congressional action; Congress plans to recess for the year in November.

Oakley said the makeup of the Saudi arms package remains open for discussion with members of Congress, and other sources said some modification of the original $1.4 billion package is likely.

A Capitol Hill source said a likely modification is elimination of 1,600 Maverick antitank missiles, which are the most important target of congressional resistance, or an arrangement under which an old-model Maverick missile now in Saudi inventories would be withdrawn whenever a new-model Maverick is sent. In this procedure, the number of antitank weapons held by Saudi Arabia would not change.

On June 11, President Reagan withdrew a proposal to sell the Maverick missiles to Saudi Arabia when it became evident that Congress would vote overwhelmingly to block the deal.

Lawmakers warned the administration that there would be serious trouble when the administration made known its plan to resubmit the Maverick sale as part of a larger transaction.

Another controversial item is the proposed supply of 12 more sophisticated F15 jet fighters to the Saudis. The administration maintains that the increasing air patrols by Saudi Arabia, including airborne protection of U.S. air surveillance aircraft over a wider area of the Persian Gulf, justify the additional jets.

Oakley maintained in her statement that the arms sales being contemplated "would not affect the Arab-Israeli military balance in any meaningful way." She added that "the administration remains committed to maintaining Israel's qualitative edge."

The executive committee and board of directors of the American Israel Public Affair Committee (AIPAC) decided earlier this month to mount a full-scale campaign against the Saudi arms sales, according to an AIPAC official.

This appeared to rebut some reports on Capitol Hill and in the administration that AIPAC was not in full sympathy with the battle against the Saudi package.