WHO WILL fare best in a world without the constraints of the SALT II treaty? Will the United States be able to build weapons more quickly and efficiently than the Soviet Union? Or will we be running free in an arms race that we may lose?

President Reagan apparently is convinced that America can win this race and achieve greater security without SALT and its limits. Thus his surprise announcement two weeks ago that the U.S. will no longer feel bound by the "standards contained in the SALT structure" and will instead respond to the "threat posed by Soviet strategic forces."

A gloomier view of our prospects in this arms race emerges from statistics gathered by the Central Intelligence Agency and the Defense Intelligence Agency. This data, summarized in the accompanying tables, shows that with a roughly equal military budget, the Soviets have been able to produce much more military hardware than the United States.

Moscow, in other words, is likely to get more bang for the buck in the arms race that many analysts predict will follow abandonment of SALT II.

This military analysis of life after SALT offers an alternative to the moralizing, pro and con, that tends to dominate the arms-control debate. And it helps answer the one question of overriding importance in the SALT debate: Will the United States be more secure with the treaty, or without it?

Consider the CIA and DIA data, which was presented three months ago in testimony to the Joint Economic Committee. The statistics show that with slightly greater defense spending from 1974 to 1985 the Soviets were able to produce a vastly larger volume of weapons.

The adjoining table marked "Output" documents this startling gap between U.S. and Soviet arms production. From 1974 to 1985, the Soviets produced more than three times as many strategic missiles; nearly 10 times as many surface-to-air missiles; 50 times as many bombers; nearly twice as many fighters; more than three times as many helicopters; more than twice as many submarines; three times as many tanks, and 10 times as many artillery pieces.

There are many reasons for this disparity: Pentagon mismanagement, congressional meddling, the military's enthusiasm for "gold-plated" state-of-the art weapons that can only be purchased in small quantities, and the Soviet push during the 1970s to match U.S. force levels.

But the reasons for the gap matter less than the fact that it exists -- and may get worse in a post-SALT era. That's because the superpower tensions that drive Soviet weapons spending may lead a skittish U.S. Congress to cut our defense budget in an effort to slow the arms race. There are already signs that President Reagan's decision to abandon SALT may have precisely that effect. Indeed, only days after his announcement that the U.S. wouldn't feel bound any longer by SALT limits, Reagan was appealing to Congress not to cut spending for the nation's nuclear forces.

The CIA and DIA data make clear that the Soviets are well-positioned for the new arms race. "Most Soviet weapons expected to be delivered to the Soviet forces through 1990 will be manufactured in plants already built and operating," the agencies said in their congressional testimony.

The future imbalance in U.S. and Soviet military procurement is suggested by the accompanying table labelled "The Future," which was prepared by the CIA and DIA before the administration announced its decision to abandon the SALT limits. The table projected that over the next five years, the Soviets would outproduce their already high procurement levels of the past five years in submarines, tanks and strategic bombers. They would produce only slightly fewer strategic missiles, fighters and helicopters, the intel igence agencies noted.

The picture becomes even gloomier when you asssume that both sides have abandoned SALT entirely. A report prepared last March by Rep. Les Aspin (D-Wis), chairman of the House Armed Services Committee, does just that.

Intelligence data cited by Aspin show that, in his words, "the Soviets have two inherent advantages that would allow them to spurt forward with force increases faster than we if SALT were undercut."

These Soviet advantages include greater "throw-weight" for their missiles, which would allow them to carry more warheads with their existing arsenal of rockets, and "hot production lines" for strategic weapons. The Soviets, for example, are already producing eight major new strategic systems -- two new ICBMs, two new strategic bombers, two new missile-carrying submarines and two new missiles for these subs. The U.S., in contrast, has only three such "hot production lines."

Aspin estimates that because of the production-line disparity alone, Soviet strategic forces could grow by 65 percent by 1989, compared to only 45 percent growth for the U.S., if SALT is scuttled.

The post-SALT danger to the U.S. won't come just from the new weapons the Soviets can build, but from the older ones they don't have to retire. Aspin notes that continued observance of the SALT treaty would force the Soviets to retire more than twice as many missile launchers as the U.S.

Military comparisons like these help explain why the Joint Chiefs of Staff, until recently, were skeptical -- on military grounds -- about the wisdom of abandoning the SALT II restraints.

The danger for the Reagan administration is that in abandoning SALT II, it could get the worst of all possible outcomes. The administration's announcement could frighten the Congress into cutting U.S. strategic programs; and it could frighten the Kremlin into stepping up Soviet arms spending. In such a world, even the Reaganites might pine for the good old days of SALT.