The 67-9 Valiant started the evolution into the Duster for 70 and has the same body lines from the door forward and still has a 108 wheelbase and dash like the 67-9 Valiant. I haven't compared the wheel wells between the two but I have had 275/60/15's on one of my 67's before and I have...
Yep and you can go at least 255/60/15's on the rear. I have 15x6.5's for my fronts from a diplomat. I will be running 15x5.5 C body wheels on the front of my other car that is manual steering.
It looks like you may have a B body axle under your car by how much extra back spacing you have left. I have 15x7's under my Valiant with 255/60/15's with a 4" back space and don't have nearly that much room on the back of my wheel.
My Valiant felt like a low rider with 13's on it and I was always watching out for speed bumps and any other road conditions more. I prefer the 15's it has now
There was plenty of room. Take a piece of wood against the drum and measure the outspacing. I had 275/60/15's on it and I think I had more room to the outside still. The 1/4's bow out on Dusters where other cars are straight
Try putting them on bare first before going through the trouble of buying and mounting tires on,just in case. I would put the jack stands under the control arms to mimic the suspension under a load and rotate the wheels back and forth.