NEW YORK -- Northrop Grumman's chairman and chief executive officer said yesterday that his company was in "active" discussions with rival shipbuilder General Dynamics on swapping billions of dollars worth of shipbuilding work involving the construction of LPD-17 amphibious dock ships and DDG-51 destroyers. "It's . . . under serious discussion," Kent Kresa told attendees of an aerospace and defense investment conference here sponsored by SG Cowen. "I think we and General Dynamics have talked about what made sense and...