The 8mm bolt just bolts the Aluminum arm to the rubber mount.
There are 2 6mm small bolts one each side of the rubber mounts, these bolt the rubber mount to the frame.
Use a 6mm Allen wrench with the short end cut down to fit into each bolt. there is not much room to get into these.
Be sure to have the hex holes CLEAN. maybe go to a car wash and blast the area clean of grease and gunk.
Some times valve grinding compound works on the 8mm hex bit to grab the hole. if it is rounded out. maybe drive in a 10mm triple square bit. The McParts stores sell these in a 4 bit set. the ridges of the bit might be enough to grip the bolt.
I also read of driving a Socket over the head of the bolt, forget the size. and the tight fit will grab it.
Drilling off the head of the bolt is another option.
If you can`t get the bolt out, remove the 2 6mm bolts on either side of the mount. then remove the 4 17mm nuts on the Aluminum arm and lift it out. then put it in a Vice to work on. might be enough room to wiggle it out.
I have an arm with a stuck bolt, I just replaced the arm from one I got at PNP.
Back in 97 before I understood how these things go together, I had a bolt that rounded out. NO problem, right? I used a Saws All and cut the arm off.
Then when I got it out I felt kind of stupid looking at what I did.
Good thing for PNP.
Charlie