Kent, back when I was "recycling" lead from backstops, I was lucky enough to find some aluminum-foil cupcake molds.
I doubt that the Teflon, or whatever the lining is, will contaminate the lead. Unless you are getting pure lead, most all of it will have one or another "contaminants" in it. Very hard to find pure Pb unless from a scientific supplies store or the like.
But, PLEASE - worry about the muffin tins being contaminated by the lead - it is a persistent heavy metal which can cause all sorts of problems if ingested. Mark the tin with something permanent, like holes drilled around the edge, or letters stamped into the metal.
About 3/4" deep is approximately 1 pound.