These cars aren't like modern cars that are half plastic and have specific grounding locations. Hemikiller listed the main critical grounding points from the battery. Because these cars' bodies are all bolted or welded together most any metal point on them provides a ground. Just make sure the metal is clean and use internal star lock washers to bite into the metal. A little aluminum or copper based anti-seize compound will help prevent rust and help with the ground. However, for a high current draw or sensitive device (like a larger stereo amplifier) I would run a dedicated ground cable that is the same size as the main power feed cable.