Clutch Fan Info

I shopping for a thermal clutch and fan for my 70 Dart (warmed over 360 magnum about 370 hp, no ac, no pb, ps, 3500 converter with a huge trans cooler in front of the radiator)

Ive got a 27 inch plus, 2 core aluminum radiator, a hi vol water pump, an ac water pump pulley for extra pump rpm s
no shroud yet

the distance from the tip of the water pump shaft to the radiator is just over 3 in

my question is, how close do I want to the fan to the radiator ?