Even if you were Van Day Truex, the year was 1952 and you had just resigned as president of the Parsons School of Design to take a shot at a private life, you will still be stunned by the enormity of such carte blanche. Imagine its happening twice in a lifetime. This September, no sooner had 74- year old Van Day Truex sold his beloved house in Provence, France to return to a life centered in the United States, then the telephone rang. It was Walter Hoving on the line again. George O'Brian , the influential design director whom Mr. Truex had chosen to be his predecessor 12 years ago was leaving. Nothing else but Mr. Truex's filling the gap would do.