You might need some special pickup ring if it is to compensate for an oversize EMG pickup rout, but in general I've not heard of this, I think normal pickup rings will cover that.
BKP humbuckers are a standard size and should fit normal pickup rings. The only place where I've had issues is with the baseplates, which do not touch the pickup rings but can rub on the pickup cavity if the baseplates on your original pickups are smaller or not as 'square' as BKPs. Your main concern would be flat or angled, straight or curved, same as with your standard pickups.
