Baby seat in an A-Body?
New baby seats are set up for newer cars.(Daughter has a 2 year old) You may want to remove the back seat bottom and then top to make a mount for the top strap. Easily done with the bracing frame work of the car.Package tray area.
As Mike said the bottom will be the center seatbelt location. A 71 should have the 3 placement belts in the rear.