3/8" is very close, but as long as it doesn't hit the radiator you're okay. Actually, being close improve the efficency of the fan. Means the air has to be fulled through the radiator.
At speed, the fan doesn't move any air. It actually becomes a burden when the vehicle speed is faster than what the fan will move air. That's why they started putting clutches on fans. I don't know what the speed is, but it's fairly slow.
