Tire width is the first three digits in the size. 225 mm is 225 mm whether it is 75 or 35. The two digit number following the / in the tire size is the height expressed as a % of the width. A 225/75 tire means that the height is 75% of 225mm. A 225/60 tire means the height is 60% of 225mm. Both are the same width. One thing that could dictate whether a tire rubs or doesn’t rub might be the rim width or back spacing.
What we tend to forget is that in 63, 64, and 65, bias ply tires were closer to 83% height to width ratios. Modern tires the same height as the originals will be wider than the originals so unless run a much smaller tire, you’re liable to experience some tire rub. To get away from any rubbing possibilities, you’d probably have to drop down to a 205 width on a 5.5” rim. That would create an unusual appearance and the rolling diameter of the tire would cause you to run the engine at higher than normal RPMs which will have a negative affect your gas mileage. If you’re thinking that you want to run that small of a tire, be sure to check the load range of the tire which determines how much weight the tires will carry safely. My 1990 Riviera uses 205/70R15 tires but it weights 600 lbs less than the 63 or 64.