Warner LeRoy, the trendy, bold entreprenuer who operates both Maxwell's Plum and Tavern on the Green in New York City, is planing to make his gastronomic presence felt here with two 1,000-seat restaurants: one in Washington Harbour, the riverside commercial development on the north bank of the Potomac beneath the Whitehurst Freeway; the other a similarly sized crystal palace that will float 15 feet above the east terrace of the National Air and Space Museum.

LeRoy, whose dramatic sense of design often seems to have been partly inspired by the cinematic flair of Warner Bros., from which he received a substantial inheritance, said last evening he was "within hours" of signing a contract with Western Development, operators of the waterfront project, to build a $7 million, 25,000-square-foot restaurant. "We're literally haggling over two inches," said LeRoy, who expects the business to open its doors in fall of 1985. Western Development would neither confirm nor deny the existence of LeRoy's contract yesterday.

It's conceivable that LeRoy will open the Air and Space restaurant that same fall, according to museum director Walter Boyne, who said the project was in its preliminary design stage and would be presented to the Smithsonian's board of regents this month. If the regents approve the project, it will be submitted to congressional appropriations committees.

"When the museum opened," Boyne said yesterday, "it was estimated that it would have 3 million visitors a year. It's been closer to 10 million. One of the major complaints has been the restaurant. It's always crowded. In February Smithsonian secretary S. Dillon Ripley and I had a conversation and both saw eye to eye on the project. Warner LeRoy, who is a well-known restaurateur, was recommended to Mr. Ripley by a friend of Mr. Ripley. He has had several meetings with us. The same architect who designed the museum is designing the restaurant. It'll be a tier of glass that will sweep around the lunar module. From inside the museum, you'll be able to see right through the restaurant to the Capitol.

"The indication of surveys that we've done," Boyne said, "is that the restaurant can be foreseen to regenerate its construction costs. The ultimate operation of the restaurant will be on a competitive basis. We're not going to take the lowest bidder, and there's no guarantee that Mr. LeRoy will be operating the place. There will be a very carefully considered contract drawn up by the Smithsonian business office that will be open to bidding."

In addition to his two trendy, expensive New York restaurants, LeRoy also operates a Maxwell's Plum in San Francisco and is constructing a 1,500-seat restaurant and cafe in the old New York Public Library building at Fifth Avenue and 42nd Street.