A person with wood-working experience and the proper equipment should have no trouble reproducing new wood parts if the old rotted parts are complete enough to be used for patterns and measurements. The new parts do not have to be exact duplicates of the old parts so long as they fit and provide the proper body contours. The wood most preferred by the body makers of the time I believe was northern white ash, although sometimes birch, hard maple or oak was used as well. Oak was often used in parts, especially cross bows, where great strength was needed. You should seek out a lumber dealer in your area that specializes in hardwood.

Check out Fisher Body Manuals from that time period which might contain sketches, or drawings of various wooden body skeletons for different GM cars.