Other than the roof and doors, 67-69 Darts and Valiants do not share any exterior sheet metal.

You are pretty much limited to 67-69 Valiant for quarter panels. If you just need from the wheel opening back, you could also check Valiant 4 doors, '67-72 (the side marker lights move around almost yearly). Trunk floor is probably the same as a Dart, but the trunk floor extensions are different. Dart and Valiant trunk lids are no where close.