I know in Semiconductor industry they use conflat, a sort of copper type metalic seal for temps. above 650F. No elastomer will tolerate above 600F no matter what the datasheet says. Don't be fooled. Gumlast has a seal that tolerates 650F for short period of time.
Temperature is the only failure mechanism for the joint. For Viton which contains about 68% Fluorine on average, one must use fluorinted epoxy to splice. However, 41" O-ring can be easily molded in one piece. I have found O-rings as large as 120" ID molded in one piece, without splice. Check out...
