EMPLOYEE WAS SETTING UP TO WELD OVERHEAD ON TRUCK BOX WHICH WAS FLIPPED UPSIDE DOWN. LOW HEAD ROOM IN WORK AREA. EMPLOYEE STOOD UP AFTER CRAWLING UNDERNEATH AND HIT HEAD ON BED ON TRUCK, KNOCK ED OFF HARD HAT AND BENT OVER TO PICK UP. IN DOING SO EMPLOYEE'S HEAD HIT PIECE OF FLAT BAR PRESENT CAUSING A LACERATION ON HEAD WHICH REQUIRED SUTURES.