''We really had a time constraint,'' explained Gedale B. Horowitz, a senior executive director of Salomon. ''And we were driven very much by technology. We had to find a building that could accommodate our needs, including major-sized trading floors.''

Much of the new electrical, air-conditioning and mechanical equipment will serve three double-height trading floors. To create the extra height, workers are removing most of three existing floors, using jackhammers to demolish concrete slabs and torches to remove steel decking and girders beneath the concrete.

After the girders are cut into sections small enough to fit into a construction elevator they will be sold as scrap for about 4 cents a pound.

In some office buildings, that alteration would be impossible, but Silverstein Properties tried to second-guess the needs of potential tenants when it designed Seven World Trade Center as a speculative project.

''We built in enough redundancy to allow entire portions of floors to be removed without affecting the building's structural integrity, on the assumption that someone might need double-height floors,'' said Larry Silverstein, president of the company. ''Sure enough, Salomon had that need.

''And there were many other ways that we designed as much adaptability as possible into the building because we knew that flexible layout is important to large space users.''

Nearly 2,000 people will be working on the retrofit project during the peak period. The cost, which is estimated at $200 million - not including carpeting, furniture and other office equipment - will come out of Salomon's pocket.

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''We made a landlord contribution to the work,'' Mr. Silverstein said, ''but Salomon's costs will go well beyond that contribution by many, many times.''

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MORE than 375 tons of steel - requiring 12 miles of welding - will be installed to reinforce floors for Salomon's extra equipment. Sections of the existing stone facade and steel bracing will be temporarily removed so that workers using a roof crane can hoist nine diesel generators onto the tower's fifth floor, where they will become the core of a back-up power station.

To help shuttle Salomon employees between floors, construction crews are adding two escalators and four elevators inside the tower. And to help adjust the floor layouts to Salomon's needs, workers are moving sections of the tower's ''core'' area, which includes pipes up to two feet in diameter and air-handling equipment the size of delivery trucks.

''This is the first time I've every seen such dramatic interior changes being made in a new building,'' said Irwin G. Cantor, structural engineer for the project. ''And the whole world is watching.''

Perhaps not the whole world, but certainly some very concerned parties. Consolidated Edison intends to protect its electrical substation stretched out beneath the 47-story tower. The only existing tenant, an accounting firm, intends to protect its services and security while construction crews work above and below its four floors. Silverstein Properties and the land owner, the Port Authority of New York and New Jersey, intend to protect their investments. And Salomon intends to move the work along at breakneck speed.

''THIS is a massive project with a tight time frame,'' said Rudy M. Pavesi, a senior vice president of Morse/Diesel, construction manager of the Salomon project. ''I cannot think of any retrofit project in the city where anyone has spent more than $5 million a month. But at our peak time, we'll be spending more than $10 million a month.''

By next July, Salomon intends to move about 2,000 employees into the World Trade Center tower, and 1,000 more employees by the end of the year.

But given the magnitude and complexity of the construction work, that schedule may be unrealistic.

''Essentially, Salomon is constructing a building within a building - and it's an occupied building, which complicates the situation,'' said John D. Spassoff, a district manager of Silverstein Properties.

Elsewhere in Manhattan, other financial-services firms designing new headquarters from the ground up have not suffered setbacks like Salomon's aborted plans for the Columbus Circle site.

Morgan Guaranty Trust Company is building itself a 1.6-million-square-foot tower at 60 Wall Street that will be ready for occupancy and bristling with high-technology equipment later this year. United States Trust Company of New York will be moving in less than a year to a tower under construction at 114 West 47th Street, where it will be the major tenant.

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''If a company can get together with a developer in an early stage, that's the best possible timing,'' said Richard Joynes, president of Hunter & Partners, a construction consulting firm in Manhattan. ''First of all, a 500,000- to million-square-foot user can effectively make a developer's speculative office project work financially, so the tenant is in a much stronger position to dictate terms of the lease. And the tenant can have features built into the space at minimal cost -rather than ripping out floors and making other changes after the steel and concrete is in place.''