You don't have to be a right-wing nut to be wary of retreads from the Nixon and Ford days. Most people, especially in Washington, understand the virtue of a fresh approach to festering problems.

Still, two potent reasons combine to favor the old boys over the new in the competition for top posts in the Reagan administration. So much so that filling Cabinet slots has become mainly a matter of the right mix, and the big fights are apt to come over the secondary jobs.

One force working against the new boys is a relative drying up of the private sector as a source of talent for government. Though hard to analyze, the development is apparent in every quarter.

The academic world is used to serve up leaders who commanded national attention and moved easily into government. James Killian of MIT is an example from the Eisenhower years, and McGeorge Bundy, the former Harvard dean, from the Kennedy administration. But since the student revolt of the 1960s, the top academic posts have generally gone to persons able to yield gracefully to their subordinates -- notably the faculty. So having been president of Princeton or Yale does not exactly guarantee making a dent in government.

Lawyers, particulary establishment East Coast lawyers, once played a central role in government. From Dean Acheson to John Foster Dulles through Cyrus Vance, they were the mainstays of postwar American foreign policy. But many of those now over 50 lost their nerve amidst the maelstrom of Vietnam and its attendant troubles. A lot of the younger lawyers, particularly on the Republican side, seem to be more into money than public service.

Business has at all times been uneven as a training ground for government. Robert McNamara of the Ford Motor Co. became a tremendous force in the Kennedy-Johnson years. Charles Wilson of General Motors counted for little in the Eisenhower era and Edward Stettinius of U.S. Steel for less in the Truman days. Increasingly, moreover, big businessmen tend to be corporate bureaucrats without a flair for success in new environments. The more spectacular successes -- in electronics, for instance -- have been made by gunslingers, too vain to work in government harness.

The second force favoring the old boys is the chancey posture of America in the world. For 15 years after the war, the United States enjoyed such manifest superiority that it didn't matter much whether a klinker served for a while as secretary of defense.

But the vortex of world politics has moved to the Persian Gulf. While the Russians can easily bring force to bear in that area, the United States cannot. The appearance of American weakness there frays ties with the European Allies who depend upon the Gulf for oil. Lack of cohesion with the Allies impairs this country's ability to deal effectively with Russia and even to manage its inflation and energy problems.

A tangled skein of delicately interconnected problems, in other words, now confronts this country. The United States cannot afford, even for a little while, to have untried persons stumbling around as secretaries of state and defense and treasury.

As it happens, a relatively rich crop of talent has emerged from the Nixon and Ford administrations. George Shultz could serve with distinction as secretary of defense or state or treasury. So -- though with perhaps less distinction -- could Don Rumsfeld. Gen. Alexander Haig, while posing a problem at Defense -- because the statute debarring military officers from the top civilian post there would have to be changed -- could step in easily at State. John Connally, though perhaps too commanding for a diplomat job, could obviously acquit himself well at Treasury or Defense. Though some might enjoy more respect than others, many, many persons could handle the Treasury job.

What counts most in picking among these candidates is how they fit together.

The trick is to strike the right balance between fighting inflation at home, on the one hand, and rebuilding American strength abroad, on the other. There is a danger of putting off friends and allies by too much saber-rattling. There is equally a peril in slighting foreign policy requirements by concentrating too hard on inflation. But the choices at the top are mainly a matter of fine-tuning and nice judgment -- not of bitter combat.

The sub-Cabinet positions are something else. The Reagan administration has prepared lists for the sub-Cabinet jobs, and many right-wingers have crowded their way onto the lists.

But will prospective secretaries of state, treasury and defense accept colleagues imposed from the outside? Not if they are strong leaders. So there shapes up, in filling the sub-Cabinet posts in the Reagan administration, a bit of a struggle.