There is a ceramic paste for high heat sealing to 1200 degrees F--used commonly by the people doing repair on steam systems(turbines, valves,joints, etc) that would work great here.The brand I am familiar with is called "copaltite".
If you have a friend in plant maintenance he might find a little.
I have often wondered about the "muffler sealants",that have been around since work began on Noah's ark, for this purpose.
Actually I think what I am seeing in your pics will seal with only a standard gasket-just retighten after a while and keep it tight.
