OK, So due to a parts shortage ( result of railroad strike I think ) There were 68 Valiants built with B'cuda rallye dashes. Slight increase in numbers/availability in that. Then there are the 68 Darts.
That dash has a different opening for Darts standard inst' panel but that could be altered for the rallye inst' panel. All those parts along the lower pad line are the same. You would still need a 68 B'cuda glove box door. No doubt the correct 68 rally dash is the best route. Good hunting