You have your cases separated... so, send the case upper to someone who can do what Cal has suggested.
Have you considerd getting a bottle of Mapp gas and heating the stud to glowing hot then with a big hammer to help it break the bond.
Clamp a good pair of vice grips on the stud and give it a good slow twist. Release and clamp tighter, then try to tighten and then reverse direction to remove.
Do this while it is hot.
Repeat the heat cycle and try to get it to start moving. Soak it with a good penetrant like PB Blaster. Try it again. See if it will break loose again...
Heat it up again Hit it with PB Blaster when you start heating and see if you can get it to break free. You really have to get it super hot a few times. Weld another nut on it better to see if you can get it to yield with a big wrench, worst case the stud breaks and you have to go the route of building it up as Calj has recommended. Having someone experienced to do the welding and allowing you to work on it after he welds it to get it out. If they are good they will not damage your cases, I believe there was a case where a welder overspoke his abilities damaging the cases and did not take responsubility for the damage.
