The procedure was to pre-heat the parts to 800F in the kiln, giving them a good soak before welding. Chris used a nickel flux rod for filler, laying down a remarkably nice fillet. Penetration looked good all around except deep in the scroll crevice where it was hard to reach. We decided to flip the scroll over and just fill that entire area with rod to ensure a gas-tight fit for the flange. The inside of the flange was perfectly aligned and no cleanup there is needed.
Thanks to the preheating, Chris said the welding was like butter. He's successfully welded cast iron before, but this was the easiest he's seen yet.
After welding he peened the welds lightly to knock off the dross and I stuck the whole shebang back into the kiln. After re-heating to 800F, I let it cool slowly to about 160F over 6 hours. No cracking observed in the finished product. Oxidation is mild and cleans up easily with a wire wheel on the Dremel. After a thorough cleaning, I'll paint it with the black VHT paint and fire it again in the kiln.
After Chris finished he let me have a go welding some scrap steel. I had a hard time sparking it up and not getting the rod to stick, never mind dragging a puddle and actually welding. This was my first time trying stick welding. Gives me all the more respect for skilled welders! Hopefully I have better luck with a MIG.