What actuates the reverse lights?
Unless they changed things between 67 dart and 68 valiant. There is a arm attached to the tube that rotates when you move the selector. If you replace the switch there is a hole in the switch, move the switch to the propper location and insert a small rod to keep the switch in that position. After you mount the switch with the selector in park you can remove the small rod and the switch will be on the correct side of the metal arm.
Go to mymopar.com for a free service manual if you haven't already