"All of the above". If a bolt is torqued properly, it won't get loose. If not, there's no type of locking device in the world short of welding it to the flywheel that's going to keep it from coming loose. I use red Loctite on clean threads, torque the bolts evenly, and forget about extra devices. Split lock washers are particularly bad, as they allow oil to creep down the threads and leak into the bellhousing through the split.
Jerry
