also one thing I am going to try before I sacrifice the secondary to the Grinder is run a few laps of 8 gauge or whatever the biggest wire
I can run in the gap between the secondary and transformer core
You can leave the secondary intact and try to sneak a few turns through the area where the filament winding is usually placed. This would mean removing the filament winding, and maybe some of the hardboard insulation. You will be limited to the number of turns, but the fewer turns, the less trouble you will get into.
Be sure to tape up the secondary wires very well as there are lethal voltages on those wires. You don't want to accidentally touch them.
If you max out the Variac and still can't get enough heat with one turn, add turns until you do get the required temperature.
Try to make very good contact with your electrodes and keep the secondary wires as short as possible, in other words, the transformer should be very close to the work. Put a clamp probe on the secondary to measure the amps and go slowly with the Variac. Happy heating.
This is high current AC resistive heating, not DC and not HF AC. Sodium filled valves, if you have them should be a very big concern
Maybe you could just shrink the valves stem diameter down by packing with some dry ice to free it up?
i know extreme cooling is used to free up close tolerance shafts and collars etc..
Just because it has a patent application or is patented does not always mean it really works.