Suggest re tightening the diff after the first run or two. Tends to loosen up a little as everything beds in. Agree that the later CR coil spring works better than the cupped Bellville washers. The B-Fast is nice, but it shouldn't be needed to keep it from slipping, more about getting it smooth. Many run stock stuff with hot motors with no problem, but they do have to be set properly and checked occasionally.
Or if it's not for racing perhaps try the gear diff from the Desert Truck I think it is. Don't know much about it myself, but it should be lower maintenance. Might require a different tranny housing.