Out on Long Island, top editors at the Garden City headquarters of Newsday would gather each afternoon recently at a mock news conference to decide the best stories of the next morning.

Across the Hudson River in Hackensack, N.J., editors of The Record already have prepared a prototype morning edition.

And up in Westchester County, executives at the Gannett paper Today are testing an edition in the Bronx.

All three newspapers are preparing for the possible death of the New York Daily News.

The News, which last year lost more than $12 million even though it has the largest circulation of any general interest newspaper in the country, was put on the sales block in December by its parent, the giant Tribune Corp. of Chicago.

Implicit in that announcement was the notion that if no buyer is found, the Tribune will fold the News, which sold an average of 1.55 million copies a day last month.

The Tribune has not set a deadline by which it must sell the News. But although there have been nibbles and exploratory talks, there is as yet no one that can even be placed in the category of potential buyer.

Competitors of the News are preparing to fill the void that would be created if the Daily News closed.

"Everybody's hoping the News doesn't fold, but no one wants to be out in the cold unprepared if it does," said a top executive at one newspaper. "But we don't want to look like sharks milling around a carcass."

Newsday President David Laventhol will say only that the afternoon tabloid, owned by the Los Angeles Times, is making "contingency plans" in the event the News folds. Most analysts expect Newsday to go after readers in Brooklyn and Queens. These boroughs, part of Long Island, would be the easiest for Newsday to serve.

Newsday sources said that the recent mock news conferences at the paper were based on a simple premise: "Suppose the New York Daily News goes out of business today. What stories do you have for a New York edition of Newsday tomorrow?"

Malcolm Borg, publisher of The Record, said that if the News folded today, he could have a morning tabloid on the streets in Bergen County "tomorrow." He said the Record already has done a prototype edition of "A.M."

Borg said the morning tabloid, if it is launched, would be separate from the 148,000-circulation Record. He said the newspaper would be "short, sweet, hopefully classy," aimed at a readership somewhere between the Daily News and the more upscale tabloids like Newsday or the Chicago Sun-Times.

He said the record is doing major market research right now and might bring out a new morning newspaper no matter what happens to the New York News.

Joseph Ungaro, executive editor of Gannett's Today, said the newspaper has always circulated in the Bronx and it makes sense to publish a second edition geared for Northern New York City. He said, "obviously" one of the reasons they're doing it now is because of the situation at the News.

The News sells about 200,000 copies a day in the northern tip of Manhattan Island and in the Bronx, the only one of New York's five boroughs that is on the mainland.

The New York Post, whose circulation has been gaining swiftly, probably would try to fill much of the void in the event of a News failure. Publisher Rupert Murdoch has announced plans to launch a Sunday edition later in the year. The Post, technically an afternoon paper, begins printing the night before, as do morning papers, and sources say more than half the Post's 900,000 daily circulation comes from its morning editions.

But the News is far from ready to give up the ghost. Warner Communications has held exploratory talks with the Tribune, as has Arthur Levitt, head of the American Stock Exchange. Levitt said it is "very premature" to talk about whether he and partner Marshall Cogan, a New York industrialist who was a partner of Levitt's when he was in the brokerage industry