the door lock on your 70 dart should be the same as on 67-69 A-Body's. if so, the lock is held in place by a thin metal "C-clip" that slides between the door lock and the door.
remove the window crank - should be an allen screw in the middle of the crank at the attachment shaft OR a "c-clip" around the shaft of the crank (that is a chevy design). remove the arm rest - two philips screws through the arm rest into the door. there might also be a chrome trime piece by the arm rest that also has a philips screw holding it on. once all items are removed from the door panel, look at the panel closely to see if there are any visable philips screws. if you don't see any, take a WIDE flat screw driver or a metal paint putty knife and insert it at the bottom of the door between the door panel and the door. pry outward. the door panel should separate from the door with a "click" or "snap." follow all around the door panel prying it from the door. remove the door panel and take the window crank and roll the window ALL THE WAY UP. you should be able to see the door lock clearly once the window is up. shine a flash light on the lock and you will see the thin metal spring c-clip. disconnect the locking "actuating" arm from either the key lock or the door lock and pry the clip away from the lock with a small flat screw driver. once the clip is removed, the lock will push out through the door.
hope this helps...