2 inches is "too far" to be effective. 1/2" is "too close" — you won't be able to get the bolts in and it may contact the radiator when the engine mounts flex. I think somewhere around 3/4" to 1" is the sweet spot. BTW, it looks to me that you really can't move that particular fan any closer to the shroud — you will risk the fan blades contacting the rim. Think about how when you rev the engine, it rocks to one side. You really would want 1/2" clearance between the blade tip and the shroud rim. Right now your fan only clears because it has the notch in the corner of the blade. If you pushed it in further it looks like you would have almost zero clearance.
