President Clinton is putting a Marine Corps general in charge of U.S. Southern Command--the first time the Army has not headed the command since it was created 50 years ago to coordinate U.S. forces in Latin America. The Pentagon announced that Clinton nominated Lt. Gen. Charles E. Wilhelm, commander of U.S. Marine Corps Forces Atlantic and commanding general of the 2nd Marine Expeditionary Force, to be commander of Southern Command. Wilhelm would be given a fourth star to lead Southern Command. He must be confirmed by the Senate. Wilhelm, 55, a native of Edenton, N.C., would replace Army Gen. Wesley Clark, who left Southern Command earlier this month to assume his new post as the top NATO commander in Europe and commander of U.S. European Command.