Don't have second thoughts. Seriously, being a scientist is one of the best careers for a person who stutters. Research involves much more written communication than spoken. On top of that, you will be surrounded by compassionate, educated people who will want to listen to the content of your speech rather than the delivery. Also, you will be so fulfilled doing what you love that nothing else will matter and your speech may even improve greatly. Do it for your speech! Guarantee you won't regret it.