Leading U.S. specialists on Soviet affairs say the new U.S. proposal at the Geneva talks on reducing long-range nuclear missiles remains basically nonnegotiable in Soviet eyes despite modifications earlier this month by President Reagan.

Reagan said he made changes in the U.S. position at the strategic arms reduction talks (START) to demonstrate increased flexibility that might then lead to agreement.

But six specialists interviewed recently pointed out that the new proposal still would require large and quite specific cuts in missile forces most valued by the Kremlin while requiring little retrenchment in U.S. forces and allowing a go-ahead on all new U.S. weapons such as the MX, Trident and cruise missiles.

These points are similar to criticism of the administration's stance at Geneva made by several lawmakers during hearings last week on arms-control policy by the Senate Foreign Relations Committee.

Administration officials argue that while the modifications may not look like much to Moscow, the Soviets have "not been forthcoming" with proposals of their own and "we are not going to negotiate with ourselves."

But "in my opinion, this proposal is not the basis for serious discussion in the Soviet view. There has not been sufficient change," Seweryn Bialer, a Columbia University faculty member recently back from Moscow, said of the Reagan plan.

"There is nothing in this proposal remotely of interest to the Russian leadership," agreed Dimitri K. Simes, a Moscow-educated senior associate at the Carnegie Endowment for International Peace.

Most of the U.S. demands remain "absolutely unacceptable" to the Soviets, said Robert Legvold, a senior Soviet specialist at the Council on Foreign Relations.

But Legvold said that one change Reagan did make--lifting the number of missiles allowed each side from the 850 in his original START proposal to an unspecified level closer to the 1,450 suggested by Moscow--was potentially important. "It moves in the right direction" and "does seem to be part" of an evolution in administration thinking, he said.

Legvold's views are especially interesting because, as he noted, he has "ordinarily been skeptical and critical of this administration's approach to arms control." He said U.S. specialists in Moscow undoubtedly view Reagan's shift as a grudging one forced on him by Congress in return for support on the MX.

But Legvold said the key question is whether these Soviet experts nevertheless will press the Soviet leadership "to take a second look." They should, in his view, because what seems "an utterly nonnegotiable lopsided proposal may nonetheless be part of an evolving American political process of change that they should find encouraging."

Legvold said that in 1977 the Soviets also viewed a Carter administration arms proposal "in the narrowest and worst context, rejected it and didn't understand its spirit. That's always the difficulty with them. But it is a hard thing to do these days in Moscow . . . or in Washington . . . to say 'Give these guys the benefit of the doubt.' "

One basic problem is that while both superpowers have thousands of missiles and bombers, those forces "are now very much out of synch," said Arnold Horelick, a former CIA analyst who is now senior Soviet specialist at the Rand Corp.

Soviet firepower is concentrated in land-based missiles, especially big multiple-warhead SS17s, 18s and 19s. These are the weapons that could wipe out U.S. land-based missiles in a first strike.

The United States, on the other hand, has much of its nuclear firepower at sea in submarines largely invulnerable to attack and is just starting to build the new, highly accurate 10-warhead MX land-based missile which is just as threatening as its Soviet counterparts.

The result, said Horelick, is that "we want them to tear down the same thing that we are now building and they want us not to build what they already have."

Reagan's original START proposal, laid out in May of last year, called for each side to reduce its number of warheads by nearly a third to 5,000 each. Each side also would have had no more than 850 missiles. The Russians now have some 2,350 missiles and the United States about 1,600.

The president now has proposed increasing the 850 figure. This is viewed in some circles as a positive move because it is meant to encourage both sides to add small, less threatening single-warhead missiles to their forces and reduce reliance on the big weapons which are so valuable that either side may be tempted to fire first in a crisis.

But Reagan has retained other portions of his original proposal which included sub-ceilings allowing no more than 2,500 of the 5,000 warheads to be based on land and letting the Russians keep only 210 of their 788 SS17s, 18s and 19s.

The experts here all agreed that sooner or later these sub-ceilings must be dropped because they require cuts of more than half in the overall Soviet missile force, which is mostly on land, and about 75 percent in the Soviets' most modern weapons. They would not affect U.S. plans to deploy 100 new MX missiles and hundreds of new Trident II missiles.

The dilemma, said Horelick, is that the sub-ceiling on the big missiles is the way the Reagan administration wants to limit the Soviets' most dangerous striking power, "but the Soviets are adamant. They will not swallow those sub-ceilings."

William G. Hyland, a senior associate at Carnegie, said "the sub-ceilings have got to go. The Soviets will not change their force structure, except very, very slowly" and the United States cannot dictate to Moscow what missiles it can and cannot have, he said.

The key, said Hyland and all of the other experts, is to allow some "freedom to mix. Let the Russians pick their own poison . . . come to their own conclusions." In other words, he and other specialists advocate that each side be allowed to design its own force within the overall 5,000 warhead level as a first step toward a more complex agreement.

Hyland said he believes that Moscow, recognizing that its own land-based missiles also are vulnerable, probably is moving, like this country, toward smaller, mobile missiles.

Ironically, the U.S. Arms Control and Disarmament Agency (ACDA) proposed dropping all the sub-ceilings in its contribution to the debate within the administration prior to Reagan's June 8 announcement, informed officials said.

The ACDA plan proposed, in addition to a 5,000 warhead limit, a roughly equal level of lifting power, or throw-weight, in each missile force. The level would be somewhere below the current Soviet level and above the current U.S. level. The big Soviet missiles now have more than twice the throw-weight of the U.S. counterparts.

Although it has also been assumed in public debate that the Soviets would reject giving up their lead in lifting power, the ACDA proposal was based on the understanding that the United States would not actually build up to the allowed throw-weight but would have the "right" to do so, officials said.

Raymond L. Garthoff, a former U.S. arms negotiator in the Nixon administration and now a senior specialist at the Brookings Institution, said another main problem with the U.S. proposal is that it does not address the question of jet-powered cruise missiles. The Reagan administration is planning to produce thousands of these missiles, which look like flying torpedos.

Specialists believe a formula can be found to deal with cruise missiles carried by bombers. But there is a special arms control problem over several hundred nuclear-tipped versions the Navy is planning to put on ships and submarines.

The Pentagon has been studying how to deal with these missiles in arms control. But one senior official said there still is no agreement on how these weapons would be used in wartime and that this is holding up assessment of how to treat them in arms control.

Some U.S. officials believe the cruise missiles, in which the United States holds a technological lead, would be a good bargaining chip to gain reductions in Soviet heavy land-based missiles.

The issue, however, is not simple. The United States, for example, has many more targets on its coasts than the Soviets, and undersea terrain reportedly makes it harder to get closer to Soviet coasts. So officials say it is not easy to agree to Moscow's demands that only short-range cruise missiles be allowed.

Finally, all the specialists said no progress can be expected in START until there is clarification as to the planned deployment in Europe late this year of new shorter-range U.S. missiles there to counter other Soviet missile forces already in place.

Those missiles--Pershing II and cruise missiles for the United States and SS20s for the Soviets--are being dealt with in companion U.S.-Soviet negotiations in Geneva known as the Intermediate-range Nuclear Forces (INF) talks.

At the moment, it appears that the planned initial deployment of the U.S. weapons will go ahead in December if no INF accord is reached before then. Chances for an accord are slim.

Because the new U.S. weapons can reach Soviet soil and the Soviets view these weapons as strategic, Horelick and others believe Moscow may then amend its START proposal to take account of the new Pershings and cruise missiles.

Whatever happens, there is a widespread feeling among the specialists that if there is to be any arms accord, it will have to be a more comprehensive one incorporating START and INF forces and possibly British and French missiles that the Soviets demand be counted but the West insists are sovereign forces not subject to negotiation.