I'm sure that these guys can write volumes about these trucks that I don't know, but allow me to offer one bit of BTDT advice - when you pull it out and attempt to remove the rest of that bolt, keep at it with the welder and the nut idea. Get the heat to penetrate fully and when slightly cooled, hit it with an impact. Do not try to drill it out. I tried to drill out an ugly MF'er in the same place after pulling the manifold, and even in a vice in a drill press that bit wanted to walk all over. It was like the bolt was cast with pockets of hardened material inside that made the bit want to walk, even after it was centered up and started. I had to go back to welding and with patience it came out.