First difference that comes to mind at under dash harnesses 67 to 68 is reverse lamp wiring. A 67 harness will have a gray plug in the lower column area. It either went on the column mounted reverse lamp switch or plugged into a extension harness to reach the floor shifter mounted switch.
A 68 model harness has these 2 wires routed to and through the bulkhead connector ( through previously unused ports where wiper wires are ) to continue on to the trans mounted switch. They're taped in with that brown wire that goes to neutral safety switch.
Second is wiper switch. When the lower dash pads were added the slot in the panel locating this switch was rotated 90 degrees ( so it didn't need to turn as far counterclockwise I guess ). Wiper wiring differences are abundant due to the different wiper systems.
There are other differences in the bulkhead connectors/wire positions relative to those hood / fender mounted indicators that a 67 Barracuda didn't have. Yes that is in the lighting harness but like I said, they moved wire positions around so something could connect to and feed back wrong to the 67 dash harness.
Your 68 Valiant dash could very well have some courtesy lighting, delay timers, and more that the 67 rally dash does not include.
In the end , what you want to do can be done but direct simple plug and play isn't possible.
