i will be ordering my wheels soon and i wanted to make sure if 4 inch backspacing is good for 8.6 treadwidth tires on a 69 cougar the wheels are 15x8, and i dont want them to stick out of the rear fender well.
i would go with atleast a 4.5" backspace. I have a 65 mustang with a 80 granada 9" and am running 4 7/8" backspacing without inner clearance probs. ofcourse i cant lower it too much or it will rub on the fender lip. a granada rear is about 1" or more narrower on each side than your 69 rear so 5" backspacing or more shouldnt be a prob since the spring perches are the same. plus the fenders stick out more, so with a 4.5" and the tires you suggested it should have no prob.