It took 10 years to put it all together, but Extell boss Gary Barnett at last locked up his dream site — the entire Fifth Avenue blockfront between West 46th and 47th streets. The real estate titan bought the holdout lot at 576 Fifth Ave., next to his larger 574 Fifth Ave., from its Korean owners in a deal that closed Thursday night, The Post has learned. The combined sites will have the address of 570 Fifth Ave. The purchase cost Extell $175…
