Oh, you building up the rims you won? Sweet. The general forum consensus seems to be the the 350 hubs are far and away the best bang for you buck hubs. Good choice, imo. Sorry, no experience with the Novatechs.
As for your question - I'm not a total wheel building novice having helped a friend of mine build up a set of xt hubs and WTB i23 rims.. and from my limited experience I's assume the same as you. That the 291 length all the way around is the way to go. That said, I'm actually confused about something you said already so perhaps I should keep my opinions to myself.
You asked: "will there be enough threads on the spoke if I use a 291 instead of 289?"
I actually thought that there were the same amount of threads on spoke, regardless of length.
If that question is painfully ignorant, at least you know to ignore my opinion