Instructions for installation of Hayden clutch fan say .......

Yeah, 3/4" seems like a sweet spot. 5/8" is good if you can get the bolts in. 1" is fine. When I see more gap than that I start looking for a spacer.

Installing the clutch fans is easy compared to a standard fan with spacer because the bolts are way in the back, behind the blades, with plenty of clearance.