Just before leaving the country Nov. 12 for London and a two-week stay abroad. William Simon passed friends a piece of news calculated to bring Treasury bureaucrats leaping from windows and Wall Street pooh-bahs wailing in despair: he definitely will be Ronald Reagan's secretary of the Treasury.

Simon has not been told that by the president-elect himself, who has been in seclusion since the election. But he is the choice for Treasury of Reagan's California-dominated kitchen cabinet. Over the past year, East Coast millionaire Simon has joined that select circle of West Coast millionaires. Among the final half-dozen men screening Reagan's Cabinet choices will be Bill Simon.

The prospect of Simon's return to the Treasury, where he was secretary under Presidents Nixon and Ford in 1974-77, builds passionate opposition. "It would be a grotesque mistake," one Reagan adviser told us, "to have Simon running the most daring, most innovative and most important economic program ever proposed by the Republican Party."

Yet, there are proponents of that program -- radical tax and budget reform -- who prefer Simon at Treasury to other names most prominently mentioned: fellow Nixon-Ford retreads Alan Greenspan and Charls Walker and Citibank Chairman Walter Wriston. Simon would be far less prone to establishment pressures.

Simon is indeed a retread of a different color. Within the dingy-gray Nixon-Ford regime, he was a scarlet stirrer-up of strife -- terrorizing aides, fueding with other Cabinet members, defying the president. In 1976, his colleagues felt Simon was playing footsie with Reagan while supposedly supporting Ford for renomination.

He was an early rider on the 1980 Reagan bandwagon, revealing his desire to return to Washington as secretary of defense. But Reagan's national security advisers insisted that Simon's abrasiveness rubbing against Pentagon brass would yeild disaster. By early October, the talk turned to Simon's going back to the Treasury.

At one recent kitchen cabinet meeting, its unofficial chairman -- California drug store magnate Justin Dart -- did not mince words. Dart made clear that Simon was his first, second and third choices for the Treasury portfolio and that there was no need for other names to reach Reagan. Whether the other kitchen cabinet members fully agreed, they did not dispute the 73-year-old, authoritarian Dart.

To Dart and other businessmen across the land, Simon is a folk hero for his unadorned free enterprise gospel preached in his best-selling "A Time for Truth." That 1978 book made Simon a national figure. When Reagan listed his economic advisers at campaign rallies this fall, Simon's name -- and his alone -- generated cheers.

To Wall Streeters, however, Simon is a former bond salesman they don't want doing a Treasury encore. Some claim his new job running a firm that manages U.S. investments for Saudi Arabia's fabulously rich Suliman Saleh Olayan disqualifies him for the Treasury. Simon's response: "Nothing I do disqualifies me for anything."

Advocates of supply-side economics pressing for a Reaganite economic revolution are not disturbed by Simon's unpopularity among the barracudas of Wall Street, the gnomes of Zurich or even the bureaucrats of Washington. What bothers them is his Republican tendency toward inflicting maximum economic pain, through high interest rates, in hope of achieving minimum inflation. That politically suicidal course was rejected by campaigner Reagan when he promised prosperity instead of pain.

Actually, Simon's record on taxation is mixed. At the Treasury, he toyed with a radical tax reduction but was staved off by the Ford White House. He has tended to tie tax cuts to rigid dollar-for-dollar budget reduction. While "A Time for Truth" ignores tax reduction, Simon's recent sequel -- "A Time for Action" -- trumpets tax cuts in supply-side language ("to restore incentives to work, invest and produce").

So some supply-siders, while preferring a sophisticated ally such as New York City businessman Lewis Lehrman, think Simon at the Treasury would stand up to orthodox opinion. He is the retread least congenial to the establishment and most acceptable to Republican politicians who deplore a Nixon-Ford restoration.

Though a veteran political infighter, Simon today may be in the wrong place at the wrong time. As Simon traveled east to London, Reagan's economic advisers were going west to California to discuss economics with the president-elect. His legion of enemies will not easily concede the post that Simon has confidently told friends is most assuredly his.