can I use lead to seal an exhaust leak>

Ha, how about the guy in front of you in the drive through back in '74 with that big overlap cam and the pig rich Holley? This would have been far downstream of any temps higher than what would re-melt or even re-fume lead, and the dangerous fumes are actually produced when you vaporize it. It's a moot point now anyway as I have sealed it with some muffler seal.