B6 and B8 are the same for the 306 actually, you're right that usually B8's are the same damping as B6 but shorter in length for lower cars, but not so on the 306. I have the same suspension setup as you are mentioning, B6/8 dampers, eibach springs and 21mm rear bars and it's really good, I would Buckley recommend the B6/8 dampers.
Sorry, that was supposed to say highly recommend!!
Sorry, that was supposed to say highly recommend!!