Google Earth no longer accepts nor uses user-generated buildings. The old ones are still available through 3D warehouse, but I don't know there's an option to download a few blocks in one shot. Not to mention all the new construction in New York city that wasn't around a few years ago. I think you may be stuck with using the old buildings from the 3D warehouse and filling in the blanks. But is there a reason why you need a few blocks? If it's just for visualization, you can make the building that you want to show, export as a kmz and load into Google Earth (privately on your computer) to generate an image of how it'll look in the city.

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