However you decide to present, it wouldn't hurt to have a letter from your doctor stating that you're transitioning. There's an official format required by the State Dept. for a gender change on a passport -- you could have your doc use that. You might not need it at all and just be waved through, but it would be good insurance in the event you're questioned.
Are you driving, flying, or...? I'd be more concerned about flying than I would about crossing the border in a car.