I do not have turbines but...
My 15 x 8 Rally's currently have on 255/60R15 on the rears and 245/60R15 on the fronts. 255/60 Would rub on my car in the front however I have heard of others running them with no issue. Main reason is pre '75 (I think) these cars were hand fit and assembled offering a variance in finished size and part placement. In 1978 all cars came with 255/60R15 stock all around. Stock size on pre '78 was equivalent to 225/70R15. I believe '68 had narrower rims. 15x7 or maybe even 15x6!
If you relocate the e-brake bracket on the trailing arms you should be able to fit 275 wide (or more) tires in the rear with no other issues I love the look of my car with the 255 vs the 225, made a huge difference.
May have helped, maybe not.
T