Once apon a time I had some wide 50's on my 68. One time I pulled off a narrow road and partilally into a drainage ditch. When I got back on the road the left side tire came down out of the wheel well and pulled the lip of the quarter panel with it.
The quarter edge cut a chunk out of the outside edge of the tire. I repaired the quarter lip with some pliers. But from then on I was careful to be sure the back tires stayed level to each other.
A long story to say that the rear suspension is not independent. If one rear tire drops out of the wheel well, the other rear tire tucks into the wheel well. If your tire is too wide, it could catch the quarter lip like mine did.
On the rear, I run no wider than 245 60's and wouldn't consider wider than 255 60.