I think you'll regret this decision. Most people converting to electric don't appreciate how otherwise quiet the brew day is about to become and then you turn the fan on for the first time. Oh crap, that's loud. If you don't think it's annoyingly loud, it's because you undersized the fan and the problem at that point is you'll have condensate dripping back into your boil kettle.
If you must go with a fan, the shortest possible path out of the space will be best. That may be punching a hole directly in the block wall just above the kettle.
If you make a hole in the rim joist sitting on top of your sill plate, you don't want to exceed a diameter more than .66 x the joist height and it has to be centered. For example, if it's a 2 x 10, you can only make a 6" hole. A 2 x 12 isn't quite tall enough to pull off an 8" unless you break the rule a bit.
The other concern is that you will need to crack a window somewhere in the basement for intake air, especially if you have a gas furnace or water heater. You can otherwise create a downdraft and carbon monoxide poisoning stuff...