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Your project is a fascinating one! I enjoy reading about it.
I don't have any Curtiss pusher experience, but I once had a Kolb Firestar with three inches clearance between the prop and the trailing edge. The vibration was noticeable but not extreme at around 2,300 prop rpm. That was stock for that airplane and a lot of them were successfully flown that way.
The folks on the Matronics Kolb list figure the more clearance the better, and sometimes added prop extensions to gain more clearance. I don't remember the numbers for sure, but think you'd be fine with six inches. Also, the more vertical distance between the trailing edge and the prop center, the better. Apparently the worst situation is to have both blades within the wing wake at the same time.
They also had a prop tip clearance issue with the tail boom, and the same clearance issues and dimensions were observed there, too. They frequently used a three-blade prop to get more tip clearance and get better prop efficiency. Of course a lot of that might simply be that the three-bladed props were better-designed props than the standard two-bladed wood prop, some of them, anyway.
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