I need some ideas on how to install this fan set up

AH 3/8 is plenty of room.
I have a 7/16 open end wrench right here that I mesured the width of and it's just a hair over 1/8 wide.
You'd have almost 1/8 on each side for safety.

That's the way I would do it.
A little grease in the jaws of the wrench to hold the nut, slide the nut in and then the bolt.