After two and a half years of absence, there is a towering presence at ground zero.  That skyscraper of muscular concrete and sinewy steel on Vesey Street is not just overlooking the World Trade Center site, it is part of it. Building No. 7, the last tower lost on Sept. 11, 2001, is the first to be rebuilt. Its emergence has surprised out-of-towners and even New Yorkers who have not been to Lower Manhattan in a few months.  Now 314 feet tall, the building is destined to reach 52 stories and 750 feet. It will be sheathed in sheer, water-clear glass, with a kinetic, sculptural, stainless-steel wall by James Carpenter Design Associates around the Consolidated Edison substation at the base.  In a speech today, Gov. George E. Pataki is expected to announce that power will start flowing through the substation by the end of the month. After the speech, he will visit 7 World Trade Center.  (Mr. Pataki may also soon announce a chairman for the World Trade Center Site Memorial Foundation, which will oversee the creation of the memorial and the cultural center. One prominent name mentioned among the possible candidates is Sanford I. Weill, chairman and chief executive of Citigroup and chairman of Carnegie Hall.)  The governor's visit to 7 World Trade Center may be a tonic for the developer, Larry A. Silverstein, who has been fighting with his insurers over the main trade center site. He holds 99-year leases on both parcels from the Port Authority of New York and New Jersey.  On Monday, a federal jury whittled Mr. Silverstein's total possible insurance claim to $4.68 billion, about $2.5 billion less than he sought. As his potential proceeds have shrunk, doubts have grown about Mr. Silverstein's ability to complete four more office buildings around the site, after the $700 million 7 World Trade Center and the $1 billion- to-$1.3-billion Freedom Tower, for which financing seems assured.  But Mr. Silverstein is no stranger to skepticism, since there were ample doubts last year that he would build 7 World Trade Center.  ''I kept hearing and hearing that,'' he recalled in an interview Monday. ''When I finally announced we had bought $60 million worth of steel, they still didn't believe. I think people finally changed their minds when the building reached 15 or 16 floors, and they said: 'You know what? He's doing it.'''  The structure is to pass the 615-foot mark in October, marking the moment when the new 7 World Trade Center exceeds the height of its shorter namesake, also built by Mr. Silverstein, where Mayor Rudolph W. Giuliani had his emergency command center. This will occur around the 20th anniversary of the first groundbreaking.  Tishman Construction Corporation, which built the first No. 7, is building its replacement. Elio Cettina and Mike Pinelli are among the Tishman executives involved in both projects. And a number of ironworkers are reprising their roles, too, said Jack Klein, a vice president of Silverstein Development.  Seven World Trade Center is to be finished in late 2005 or early 2006, with 1.7 million square feet of office space. Apart from his own company, Silverstein Properties, Mr. Silverstein does not yet have tenants. But in the time-honored tradition of developers, he said there was ''considerable interest,'' particularly now that there is a tower to see.  Already visible inside the tower is a hallmark of what is supposed to be its great durability and safety: a concrete core around the elevator shafts and fire stairs, two feet thick in most places. The stairways are ample, five and a half feet wide, and the landings are deep enough to fit a person in a wheelchair while others pass on the stairs.  ''A firefighter carrying gear could walk up while people are walking down,'' said Nicholas Holt, an associate partner at Skidmore, Owings & Merrill, which designed 7 World Trade Center, working with the structural engineers at the Cantor Seinuk Group. (They are also Mr. Silverstein's architects on the Freedom Tower.)  The surprisingly spacious lobby, sandwiched between two banks of Con Ed transformer vaults, is framed by steel-jacketed columns five feet in diameter. Outside, Greenwich Street will be recreated as a driveway for taxis and limousines but not as a through street. Across the street, Mr. Silverstein will build a 12,500-square-foot public park designed by Ken Smith.  The principal art in the lobby will be a 12-foot-tall wall of glass and light-emitting diodes by James Carpenter and Jenny Holzer, an artist known for using epigrams in her work. ''What we did insist upon was the choice of the words,'' Mr. Silverstein said. ''I wanted something uplifting, positive; that talked about renewal, talked about America, talked about freedom, talked about what our values are about.''
Construction on 7 World Trade Center reaches about half its eventual 750-foot, 52-story height, confounding those who doubted if Larry Silverstein would build it; Tischman Construction Corp, which built the No. 7 destroyed on Sept 11, 2001, is building its replacement; No. 7 will be structurally reinforced and incorporate many safety features; is scheduled for completion in late 2005 or early 2006; photos (M)