295/55/15s
If a 295 is just Barley Possible, Then i am gonna have to try.
I dunno about Dusters cuz the last time I had one was 1978.
But I know about 68 Cudas. A 295 fits,(yes, fits), easy. With,as Blu says; a spring relocation, a lip-roll and center it up.
Now, let me tell you about 325/50-15s.
Those guys just barely fit, by Blu's definition. But it also took pounding of the inner tubs in various areas and moving the axle back about 3/8".And no, I cannot fly around the turns with them in there. As the body rolls over, the inner sidewalls rub, and theres no more room on the outside and still have them "in" the tubs ,at my lowered ride height. My rear is not quite as low as Blu's; maybe a half inch higher when the 295/50-15s are on.325s are about 1.2 inches taller in diameter' 27.8 vs 26.6.
That said, in normal driving they rarely rub.And it's always on the inside, where the sheet metal is very smooth, cuz I made it so. Sidewalls make pretty good brakes though :(