Body language can really help with impressions and other voices. You need to feel like you're American.
Gain a few hundred pounds and see if that helps.
Well, seeing as the presentation is sometime next week, I doubt I could gain that much weight in such a little amount of time even if that was a serious suggestion.
Go watch the movie version of the novel. I think there is one with Gary Sinise (Lt Dan). Just parrot what you hear. I'm not sure if the specific monologue you chose is in the movie or not, but you can still use it as a way to listen to the accent.
It isn't a monologue from the novel, because there aren't any as far as I can remember. Regardless, we had to write our own monologue based on the characters of the novel, so the lines I use aren't actually in the film - hence I can't just imitate the movie versions of the book.