Personally I think it's less about trying to "appear" confident, but rather building that confidence within yourself so it invariably becomes real. I'll go into situations where I'm talking clearly for minutes on end, and then comes a block, but I do my best in acting like nothing happened because NOTHING did happen other than me stumbling over my words. People will see this and respect you more for it, for they will see how much you don't care about the stutter in those moments.