The fan spacer is 2", and as far as I know it's been on the engine throughout its existence. As for optimal cooling, I've had no overheating problems with the setup. Even in heavy traffic on a day like this (+30C) the needle just barely goes past the middle marking on the rally gauges, which I assume is about 210F.
The fan actually sits about 1/2" into the shroud, although fromn the angle the pic is taken it doesn't show very well.
