Radiator Fan Spacing

Pulled out my old 4 blade OE fixed fan and replacing it with a flex fan. The old fan had an approx. 2" spacer on it with really long bolts and the fan itself was only about 2" away from the radiator. (had to remove radiator to remove bolts). I don't think the spacer is original and he shroud is long gone. So what is the proper spacing between the fan and the radiator?