Here is 345 + 40lbs of chains for 5 15/16 reps. Couldn’t lock out the last one because if you hear my brother pulled into my driveway yelling for me to move the car, breaking my concentration. But looking at the video I think my ass rises too early.
It does, your hips are too high, coming up too early, and you are rounding your back.
The deadlift isnt two movements, its one.
You start with the bar touching your shins and shoulders in front of the bar. As you start your lift your head moves up a little, but still focusing on the ground it seems. Everything so far is wanting the lift to move forward and not backwards. I would read the T-Nation article above and drop the weight and work on mechanics. On more of a positive note, you have a nice SLDL.