The physical methods used in the restoration (or rebuilding) are not the only things that need to be preserved.
Someplace the thought process, the determination process, the planning process that occurs before the physical work needs to be documented and preserved. Why was a particular car selected as a project?. What is the goal of the project -- complete rebuild, selective replacement, repaint, etc.?
Then during the project what is discovered about the construction and repair/damage history of the car? How is this affecting the scope of the project? How is this different from the original planning? What compromises are being made during the project?
Look at and learn from the methodology used in the preservation, etc. of small artifacts, larger movable artifacts (like early airplanes at the Smithsonian), and the immovable (usually) historic buildings and houses.
Brian Norden