This is an interesting discussion and most will decide on tyre/wheel sizes based on the look they want. But if you want to drive fast on the track in a light car like the GT40, don't fall for the wide wheel trap. Rear wheels any wider than 10" with correct size tyres will never work well because they won't heat up to a good working temperature. There are a lot of over-tyred historic race cars around which have this problem. If you're ever in doubt about this, get a good tyre temp gauge and check it out. Of course none of this matters on the road because you'll never work the tyres hard enough to get them anywhere near their optimum temperature. Or if you've tried you may be reading this from gaol (jail!)