There was no Ford Ranger with a 2.9L V6 in 1968. The Ford Ranger pickup truck didn't exist until 1983, and even then, didn't initially offer a 2.9L V6. The 2.9L Cologne V6 was introduced later in Ford's lineup.
You may be thinking of a different Ford vehicle or year. To get the correct firing order, you need to specify the *correct* make, model, and year of the vehicle.