I do have a 65, and your right its about an inch. You would have to remove the bump stop, and take the inner fender to the frame rail. But an inch is an inch! I agree for the additional inch it's alot of work. Still curious if anyone has done it for any year Mustang.
Between a subtle mini tub and a subtle (entire quarter panel) flare you could have an additional two inches per side while looking fairly stock. I figure that would let you fit a 295-305 back there. Subtle is what I'm looking for. I want to make people scratch their heads saying "How the hell does that tire fit?"
I have a 98 Cobra seat fit in the back of my mustang that is narrower then the stock seat. Even though it's much narrower (approx 3") I would still have interference with the side panels and the inner wheel well if I mini tubed the car.
I'm just reaching out for any creative solutions.