LEONA`S TRIAL ISN`T OVER

It was testimony, not testimonials, that certified Leona Helmsley`s character. But Bantam Books loves her, nonetheless. Mere minutes after the dictatorial hotel queen`s conviction for tax evasion, Bantam okayed a quickie paperback. In three weeks, watch for ”The Queen of Mean: The Unauthorized Biography of Leona Helmsley.”

HEAD `EM UP . . . Shades of ”Lonesome Dove.” In Roundup, Mont., 3,500 cattle and 105 drovers on horseback are mooing and whooping in preparation for an Old West cattle drive to celebrate Montana`s statehood centennial. It`ll kicks off Monday. ”This drive will not be of the Hollywood cowboy school,” said Stan Lynde, creator of the old Rick O`Shay cartoons and originator of the 6-day hoofalong to Billings, about 60 miles to the south. An unabashed lover of his native Montana as God`s Country, Lynde promises an old-fashioned epic, complete with dozens of horse-drawn wagons. But media spokesman Cal Winslow concedes some modern touches, including cowpoke portable toilets and a helicopter, perhaps to herd stray reporters. Still, city slickers beware. Ubiquities of our era otherwise are banned-including cars, motorcycles, boom- boxes, baseball caps and tank tops. At the end of the trail Sept. 9, 100 Texas longhorn cattle will be auctioned. Okay, boys, moooo-ve `em out.

BRANDO: THAT`S IT, I QUIT Marlon Brando, the two-time Oscar winner who fled to the South Pacific nine years ago, says he`s calling it quits-from acting. ”This is my last day . . . I wish I hadn`t finished with a stinker,” mumbled the 65-year-old star, who has nothing good to say about his final big-screen gesture, ”The Freshman,” which he and Matthew Broderick just finished filming in Canada for Tri-Star Pictures. ”It`s horrible,” said Brando. ”It`s going to be a flop, but after this, I`m retiring. I`m so fed up.” Brando won his first Oscar in 1954 for ”On the Waterfront.” In 1973, he won another, for ”The

Godfather,” but he refused to accept it. That same year he turned in his last creditable performance-as the disillusioned man in ”Last Tango in Paris.”

After collecting several million dollars for a cameo appearance in 1978`s

”Superman” and then bombing big and bigger in `79`s ”Apocalypse Now”

and 1980`s ”The Formula,” Brando became a screenstar recluse. In his exit interview in Toronto, he confessed that he never liked acting, anyhow. ”But there was nothing that would pay you that kind of money for that kind of foolishness.” As for quitting, he said, ”You can`t imagine how happy I am.”

HEAVY METAL RELIEF No doubt it`s a case of arrested development: Guns N` Roses guitarist Izzy Stradlin is free on $5,000 bond after his airport arrest in Phoenix-for peeing into the galley of a USAir jetliner. Stradlin, whose real name is Jeffrey Dean Isbell, doesn`t deny the incident. But his defense apparently is constitutional impatience. According to Geffen Records publicist Bryn Bridenthal, Stradlin grew angry at waiting to use the plane`s toilet and

”relieving himself in the galley was just his way of expressing himself.”

Should a judge be moved to impose community service for allegedly interfering with an airline crew, a few dozen hours cleaning latrines probably would be more instructive than spanking the poor kid.