I was running 275/60-15 on my 68. I had to trim the sheetmetal, which I wish I hadn't done. It's been so long also, but I had bought the axle assembly from someone and I think it was from a late 60's B body, but the wheels must have had more backspacing. I think they were 7" wide wheels, so the tires were probably pinched. But they were on there.
I have 255 now with much less compromise and I think looks just as good.