I have a customer looking for some studs for a flange that they say could reach 1800deg F. What would you use for that?

At 1800F you have a couple of options. In a steel variety you have alloy 330 studs. This will be oxidation resistant and stable at the 1800F temperature. Also, you should consider Inconel 600 fasteners. This will be a more robust solution providing additional corrosion resistance and strength. The cost difference is usually minimal which makes Inconel 600 fasteners a good choice.