Workmen began at daybreak Wednesday preparing to move the "Spruce Goose," the largest airplane ever built, from the harborside hangar where Howard Hughes left his creation after its first and only flight 33 years ago. Relocation of the flying boat to a temporary sanctuary 300 yards away was scheduled to take six to eight hours. It was to have gotten under way at dawn, but an hour later the huge ship was still at rest while scores of specialists took extra pains to make sure the plane was not. damaged during the move. More than 200 technicians, four tugs, and the world's largest floating crane were involved in the move.