There is some info in this thread.
Thread size rear Spring Adjuster
Be aware that setup is relying on the Shock bolt (single shear) to support the the suspension as well. I'm not fond at all of single shear in critical areas and definitely not in boosting the load of a fastener designed for shock loads to one that supports the vehicle. Thoughts of going through a sweeper at speed and the bolt breaking is enough to prevent me from doing that mod.