I don't know what the factory used, but I think I remember using something like 7/16" coarse thread bolts on mine.
I bought my skid plate from the Ford dealer because mine was not optioned with one and I just had to have one, like all the big kids of course.
I just found what fit the existing holes in the plate, then drilled and tapped the frame because it looked stout enough from where I was lying.
I've never remembered to look at the frame on a factory equipped model to see just how they did it in all these years.
Mine has stood the test of time and Rubicon though, so it's worked out well just using the size that fits the holes.
You just wanting to know what works, or do you want to do it like the factory did for accuracy?
Paul