I think AndyF setup his brembo/viper brake conversion to account for some of the backspace of the mustang spec rims.
The Dart have about the same room in front as the Barracudas and should be the same as a Valiant.
Now in the rear the Dart lacks room that the Barracudas do. I think the Valiant also has that slab sided skirt design. I don't know/remember what the red 68 valiant has done for rear tire fitting. I bet it's at least got the quarter lips rolled. Maybe clearanced out a little (big hammer).
In the front with late model large bolt pattern you can fit the 275/40/17 on 17x9 rims with 5.7 backspacing. Might need some small spacers if you have slight tie rod inerference. You will have to roll/cut a 1"-3/4" x 3" triangle out of the lower front fender lip. Also with the dart you can move the lower fender brace forward and that will move the fender out for more clearance (I've done that on my 68 Dart). You can even redrill a new fender brace hole forward of the factory one for clearance.
Read here for 17 5.72" backspace rim fitting.
http://www.moparfins.com/CLAIRDAVIS/1969_front_end.htm
http://www.moparfins.com/CLAIRDAVIS/Cobra_Wheel.htm