I've lived stealth since I was 16. High school, work, socially etc. I met and briefly courted a chick back when I was 16, she knew nothing. It was short lived, I moved away, and 11 years later I'm back in the same state again. She had found me on facebook 6 months before I moved back to our home state.
The first 4 months back, we just hung out a lot, and were basically "friends". 2 months ago I decided it was probably time to 'spill the beans' and told her my back story. I already had a good feeling, she would be cool with it. And sure enough, she was. I really don't think if we weren't FRIENDS first, for a little while, she would of been as understanding as she is. She's generally a very open minded person, but I think the fact we got to know each other so well beforehand, definitely played a part in it.
I've had a few long term relationships, with girls that didn't know about me right off the bat. It's always been easier to be friends first, and kind of ease into it. Let her get to know YOU, for you, and not your trans status. It sucks knowing you're a good man with a lot to offer, but having to worry if a girl is going to dog you out for your genitals.
If you really like her, my advice would be to hang out, get to know her, and let her get to know you. Feel her out, and when you feel it's right, let her know then nuts and bolts.