Every one of my cars has extra ground wires on them from old to new. You should see them they are 18" 4 gauge cables I found on a mothballed, now scrapped, Coast Guard icebreaker. Go from engine to chassis.
The ground wire was changed for the 1970 model year. There is not a separate wire from the block to the chassis. The ends of the ground wire attach at the battery and the front of the block behind the alternator with an additional lug that attaches to one of the regulator mounting bolts to ground the chassis.