just a hunch, but I would get it as close as possible. You want air coming through the radiator, not from engine compartment. The radiator is open to allow air to come through. I know what you are getting at, like putting a portable fan up against the wall and no air will flow. Not so with the radiator. Hope that helps
I would say 3/4 " to 1" would be good. Hold a dollar bill in front of the rad opening with the car running and let go.... the pull from the fan should suck the bill up against the outside of the rad. and hold it there, if it does then you are good to go...
On my 76 I have a good 4-5 inches between fan and radiator. The fan just sits inside the shroud, and that's with a 2-1/2" spacer, but it pulls plenty of air. I think I'd like it to be a little closer, but it is doing the job. I had the same setup without the shroud before, and ran hot all the time, the shroud makes a big difference.
The comments about the fan shroud are good....I am so used to thinking about BB applications that I forget other motors have more room! Half in/half out of the shroud is good advice..should do the trick. good luck.