KITTERY — Aided by strong tides, construction crews installed a lift span on the new Sarah Mildred Long Bridge Wednesday, a final step toward completing one of the biggest and most expensive bridge construction projects in Maine’s history. A 4-million-pound concrete span the length of a football field was floated over the Piscataqua River on two barges Tuesday and maneuvered into place at the center of the new bridge that connects Kittery with Portsmouth, New Hampshire. The operation was timed to take advantage of intense tides in the Piscataqua River. The span rose into place with high tide Wednesday morning and dropped on bearings on the bridge’s lower deck as the afternoon tide went out.