I broke the bolts off mine when trying to remove it. I ended up grinding them flush, using a punch to dimple the center of the bolt, and drilling them out and retapping the hole on the ones that face the ground.
On the ones on the side. There is a nut welded to the inside of the frame. I smacked the left over bolt until the nut inside broke free (rust in that area helps, if your frame is clean, probably wouldn't work) but thre is an access hole you can get a new nut on a wrench and back in there.
I have had almost zero luck with easy outs and specialty extractors over the years. Mine came from Chicago and everything was froze on way to much for meer morals and their tools.