I have a '67 Dart that has factory seatbelts and turn signals, but not headrests. The '70 I bought new back in the day had headrests, but not the shoulder belts. I also believe 1970 was the year passenger cars (but not trucks) had to have locking steering columns, mine did. The floor shifter (manual) wasn't interlocked so it was necessary to push a button to release the key/lock the column. 3-in-the-tree models had to be in reverse to remove the key IIRC, automatics in park. Those early shoulder belts were 2 separate belts, with separate buckles in the seat.
Speaking of turn signals, I remember the Sears catalog offering aftermarket turn signal kits for older vehicles, the switch assembly attached to the steering column with hose clamps. I seem to recall them offering a self-cancelling version as well, not sure how that worked.
A lot of vehicles of that vintage the latching pawl on the shoulder part of the belt was overly sensitive and would latch up when you went over a bump requiring you to lean back a bit to release it. If you hit another bump before it released it could strap you way back into the seat.