It was a nightmare. I tried a torch, cold chisel, air hammer/chisel, hacksaw, and recip saw, all to no avail. I ended up using a series of chisels as wedges, and separated the LU from the upper. In doing so, I had to bust the casing where the other bolt (up behind the trim tab) held the two together. I'll have to have it welded back together.
Once I separated them, the trim tab came out, and I was able to see that the hex key bolt head was stripped, and that's why I couldn't get the bolt out.
I think this is gonna get expensive....