Is it an LA (Van Nuys) built car or a Norwood car? If an LA car, the underbody was originally black and much easier to blend in and look original than the red oxide Norwood cars. I don't see any red oxide in the photo. Like Steve said, it depends on what you want it to be. You may want to do a Google search on Jack Seymour 1970 Z28 as he had a low mileage 70Z that he did and extensive cleanup on without disassembly.
Never mind...here:
https://www.google.com/search?q=jack...w=1680&bih=960
He is also a member on this forum JacSey 70Z
https://www.yenko.net/forum/member.php?u=42384