Your definitely stronger then the weight your using, I estimate with good form your maximum to be mid 500. Your pulling with your back almost entirely, your legs go first then the weight is still in front of you then you stiff leg the weight up. This is very common when you first start off with deadlift though.
What might help you is to pull the weight closer to your body, the weight tends to drift away from you.
Here is a perfect deadlift for reference point exrx.net/WeightExercises/ErectorSpinae/BBDeadlift.html
with maximum weight it will not look perfect though, notice how the weight is close to him and both the legs and back move together? Your legs go first then your back goes which makes it harder.
Your long arms will help tremendously with the deadlift, and it will probably be your best lift and you'll be over 600lbs in no time.
Overall your form is not that bad at all, just doing the movement some more will get you more used to it.