I know I want to smoke at a low temperature for longer, but can I over cook the brisket? I am looking to have it on the smoker for around 16 hours, is this overkill and will it dry out the meat?
If brisket is cooked too long, or at too high of a temperature it can dry out some. Try to keep the smoker temperature at near 225 degrees, and smoke the brisket until its done to the stage you want it to be.
You'll know when it's done by using the fork test. Stick a regular table fork into the meat and give it a twist. With practice you'll be able to tell when it just right.
With briskets you can't go exclusively by temperature. One brisket might be falling apart tender at 188 degrees, and another might need to reach 195 before it's that tender. I think it has to do with the amount of connective tissue in the meat, or that the connective tissues of some steers is just tougher than it is in others.
It always helps to baste brisket several times as it cooks. Another way to prevent drying is to wrap it in foil after it's smoked for several hours.