Yup, makes a difference. With an 8.75 using BBP axles and brakes you actually want more like a 15x7 with 4.5" of backspace to maximize the tire width.
And the pre-'70 cars tend to run a little smaller on the quarter to inner fenderwell measurements in the back, so on some cars fitting a 245 is more of a problem than on others. Gotta measure the spring to quarter width for the car you're trying to mount the tires on to be really sure. Most can manage it, but some are a lot tighter than others.