Get the bushing working right, AND use a cotter pin
As has already been said, make it so that the bushing fits properly -- bushing is held rigid when the bolt is properly torqued, and the bell crank rotates freely and smoothly around the bushing.
After having done all that, it's still not a bad idea to use a drilled bolt + castle nut + cotter pin.
This is a nice safety measure, just in case at some point in the future the bushing stops working properly due to gunk, or corrosion, or any other reason that would cause excessive friction between the bell crank and the bushing, which in the extreme case could cause the bushing to rotate.
This is improbable, and some will argue that a cotter pin arrangement is overkill for this situation. Maybe. But, so what? It's an extra safety measure on a safety-critical component, and it doesn't really cost you anything.