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

Personally I would slot the side of that channel and slide a nut in there on an open end wrench.
Is that what you said about being really close to the core?
You would have to be careful, but it looks doable with a moto tool using the cutting wheels.

Maybe 3/8 of an inch. Yes it's doable, but I will have to be careful.