The foundry I work at uses cutting powder to oxy-fuel cut off 4"-18" risers from the casting. We air-carbon-arc after cutting to get close to net shape before grinding or machining. I don't know if the flux works the same as the cutting powder, but as long as it adds enough iron to the kerf of the cut to get the oxygen-iron exothermic reacion going, it should work. I would think that the exothermic rods would take quite a while, but I have never used ones, so I can't say from experience.