Keep in mind, 67-69 Valiants may have a slightly different size wheel well than 67-69 Darts or post-69 valiants. Just a thought. Time to take the ol tape measure out.
The wheel tub is basically the same. The wheel openings are higher than the Dart counterpart, so you have more leeway with the section width because the quarter lip is higher. You can run the same wheels as on a Dart, maybe one size wider depending on the final ride height.