i know i'm a bit late, but if you do this mod reverse the bolt in the spring pack as well so that it's captive, they have a habit of falling out and peen the thread on it to stop the nut coming off e.g ...... hit it with a hammer
mine 2012 16-491 ob not under slung brought second hand. I have under slung this morning in back yard it is a big lift 120mm higher had to reset the tow hitch on the sr5 so good heading to Tenterfield on Thursday so will see how it tows