President Reagan and his top advisers are scheduled to meet Wednesday to decide whether the United States should continue to comply with the unratified SALT II treaty, and proponents of breaking the limits as a response to alleged Soviet violations have run into unexpected opposition from the president's military advisers.

The symbolic and political aspects of Reagan's decision equal or outweigh strictly military factors. By the meeting Wednesday, at least 88 of the 100 members of the Senate will have staked out positions in letters to Reagan, and most of the NATO allies will have been heard from.

A new factor in Reagan's decision-making is a U.S. intelligence report that the Soviet Union early this month flight-tested a giant new intercontinental ballistic missile. Analysts who have expected such a test described the weapon as the successor to the SS18, the largest Soviet ICBM.

If the size of the weapon exceeds the SS18 by more than 5 percent, it would be defined as a "new missile" under the terms of SALT II. The administration has charged that the Soviets already have deployed two new missiles rather than the one permitted by the unratified treaty, which both sides have pledged not to undercut.

The administration first faced the question of SALT II's limits last summer. Then Reagan chose to "go the extra mile" for arms control and dismantle a nuclear-armed submarine to stay within the limits of the pact. This time, the decision appears more complicated.

"We have not yet come up with a plan," said a key Defense Department official who has spent 10 months exploring "proportional responses" to Soviet violations. With some bitterness, he said the Joint Chiefs of Staff "are reluctant to change their budgets to fund anti-SALT violation solutions."

The plan receiving most attention would place two 20-year-old Poseidon missile-firing submarines in drydock rather than dismantle them when a new Trident submarine goes to sea next month. The 24 multiple-warhead missiles on the powerful new sub would put the United States over the SALT II limit unless the missiles on the older subs are dismantled to compensate.

Just a month ago, the prevailing sentiment in the administration was to drydock the two submarines for a year rather than dismantle them. This halfway measure would not fully comply with the SALT II treaty, but it could be reversed if United States and the Soviet Union reached agreement on alleged violations or a new arms treaty. If not, the submarines would be sent back to sea as missile-firing systems, after overhaul.

But the politically palatable drydock plan ran aground against hard facts of military planning and budgets during a series of interagency meetings.

"The Navy does not like the idea," said a source close to the issue.

The two older submarines affected, the USS Nathan Hale and USS Nathaniel Greene, are due for major overhaul, which normally would keep them out of service up to three years. Since submarines of this class have an active life of about 30 years and these two have been in service 23 and 22 years, respectively, the boats would have few operational years left if they are overhauled and sent back to sea as ballistic missile subs.

Officials said Navy contractors are waiting word this week on whether to begin dismantling the missile systems aboard the two submarines as authorized by earlier Pentagon budgetary and operational programs. Presidential national security affairs adviser John M. Poindexter, a Navy vice admiral, is quoted as saying in administration circles that between the requirements of Navy contractors and the convenience of a U.S. president, contractors usually win hands-down.

"The Navy has done its military planning on the basis of the SALT II treaty," said an exasperated policy-maker who insisted that the United States must find a way to "send a signal" to the Soviets about treaty violations. The official said he has tried repeatedly -- and unsuccessfully -- to learn what the Navy would have planned for the Poseidon submarines if the treaty did not exist.

Reagan is scheduled to meet on Wednesday with Defense Secretary of Defense Caspar W. Weinberger, Secretary of State George P. Shultz, Arms Control Director Kenneth L. Adelman, among others. He is expected to make a tentative decision after that session and then send arms-control adviser Paul H. Nitze to Europe to consult NATO allies before announcing his decision by April 25.

Nitze's report could affect the final decison, sources said. Similar consultation with allies by Nitze and arms adviser Edward L. Rowny recently brought about major changes in the U.S. negotiating position on intermediate-range nuclear forces.

The requirement to address the issue grows out of Reagan's decision in January 1984 to charge the Soviets with violating SALT II and other treaties, a departure from the quiet diplomacy on Soviet compliance questions of the Nixon, Ford and Carter administrations. Most of the alleged violations are not considered militarily important, but the argument for some response is strong if they persist or grow, as in the case of the potential third Soviet "new missile" in the recent test.

Options for responding have been discussed in the administration. In addition to submarine missiles, they include:

*Seeking additional funds from Congress for the 50 MX missiles that were cut from the administration's original strategic modernization program. This is favored by the Joint Chiefs of Staff, but in the year of the Gramm-Rudman-Hollings budget law its chances seem small.

*Deployment of three-warhead Minuteman III missiles in silos now occupied by single-warhead Minuteman II missiles, an action that would breach SALT II numerical limits. This would be "cheap, quick and easy" but of doubtful military significance, said a senior civilian policy-maker.

*Deployment of air-launched cruise missiles on bombers in excess of the 120 aircraft permitted under SALT II. This limit will be reached this year.

*Accelerated funding of the single-warhead Midgetman missile, which would be a second U.S. "new missile" and thus exceed SALT II limits. But the Midgetman is controversial in the administration, and new money is scarce.

*Encoding U.S. missile test data parallel to "excessive" Soviet encoding of its data. It is doubtful this would deny much information to the Soviets, and it would lose "the high ground" on a secrecy issue for the United States.

*A rhetorical statement abandoning the U.S. "no-undercut" policy regarding SALT II in view of Soviet violations. In some formulations the United States would continue to comply with the 1972 SALT I treaty. Such a purely political action would generate powerful opposition from the Soviets, the allies and Congress but would have little impact on military reality.