I would say just plain JB Weld. Supposed to be good up to 500F, if I recall. Haven't used extreme heat version, but I did make a bead of conventional JB around my muffler/exhaust pipe junction, and it's prevented any of the sooty blowby that alerted me to seepage a year ago. Still solid and tight. Have seen it used by fairly reputable body shops, as long as no flexing is anticipated, such as a firewall. Much cheaper than panel bond!
