How close should my fan be without a shroud?

With a shroud you want half of the fan blades inside the shroud and the rest outside of it, distance to radiator with a shroud is mostly irrelevant, think of like a 1970 Monte Carlo with a small block. Without a shroud you want the fan as close to the radiator as possible, obviously where it will not touch with engine movement, and where you have room to put it on on take it off, you need enough room to screw in and screw out the bolts. 3/4" of an inch , as stated above should be good. IMHO you should always run a fan shroud, cools much better.