According to the fastener guide the bolts aren't anything special...just short 7/16" bolts. It looks like there are some push-on style bolt keepers/locks but they just hold the bolt inplace.(they don't hold it from spinning) If I had to guess, I think the bolts are installed on the bumper brackets. The entire assembly was held inplace while someone tightned the nuts from under the car. It was probably done right before "body drop". Unless someone has pictures showing the bolts being installed from the x-member to the outside, this is how I'm going to do it.
I'll have to go see if it looks like the mufflers will be in the way though. I didn't want to have to get the bumpers done right away but I may have to...the rear one at least.