The mosque is actually next-door to the site where the cultural center is planned. As for distance, I suppose that it depends on how long you consider a "block" to be. The site is apparently 2 blocks in one direction and one in another from the nearest part of the WTC - WTC 7. It is further from the former site of the twin towers (WTC 1 and WTC 2). In some opinions, "ground zero" is the place where the towers stood; in others it is anywhere in the city where debris way have landed. Take your pick.