I don't know a lot about brazing yet, but I've watched my dad do it a few times, oxy/acetylene hot blue flame to heat the metal to cherry red and touching the rod to it to melt the brass into it...that's GOTTA be hotter than any exhaust is going to get.

unless you run stupid lean!

but, if you're really worried, maybe look for someone who can weld cast aluminum up with steel rod.