The force of the blow seemed to rock the building, according to office workers above and below the 78th and 79th floors. So great was the impact that one of the motors of the twin-engined bomber ripped through seven walls on a continued due south course to tear out the 33d St. side of the building, shoot across 33d St. and land with a final explosion on the roof of a 17-story building at 10 W. 33d St. Miraculously, no one there was hurt.