Sorry for long post...
Well your form on the x5's is not bad at all IMO. You hips wanna raise a slightly, especially as you get nearer max stress, but that can happen and can be kinda normal for most people's 1RM pulls.
The #470 is most likely just to much of a jump, so check your ego at the door as your #365x5 does not really translate to #470x1.
It was hard to tell how close you are getting to the bar, but the closer you get your shins, the closer the weight will be to your center of gravity and thus the less the bar will want to sway out and you turn it into a hard low back SLDL.
A few things you might want to do, as I had the same problem as you going to singles.
1) Work on keeping your form constant, no matter how much weight you have on the bar. ie: if you feel you hips shoot up, stop, reset and re pull. This will make a habit for you to keep the ORDER OF MOVEMENT in line with what should happen. It is all one fluid move. As your straighten your legs keeping the hips down somewhat and then hips power forward, the bar comes straight up over the knees and you are erect. Like your first rep with #365. (There is a good paragraph of this in Rippetoe's Starting Strength.) where people think they have weak hams, but are not as weak as they think just not moving the body in proper order. The legs DO NOT straighten first, they straighten as the hips come forward and up and the bar moves up at the same time with the legs straightening. Keep things in order and you will be surprised how strong you can become and really are.
So, even if you are hitting a heavy single, make sure the order is the same.
Lastly, it can help some trying to keep your shoulders from drifting forward, as you almost wanna feel like you are wedging yourself backwards a bit as you pull. I hesitate somewhat to say too much, as you don't want to get caught up in overthinking it too much. You wanna get down on the bar with a "feel from habit" and pull hard, after you take all the slack out of the arms and body and are tensed to go.
2) It does not look like your power off the floor is all that bad, and again most likely in line with your actual 1RM, which is not #470 "yet" but you'll get it and more if you keep at it.
Impatience is one of the worse things when trying to improve strength. Be honest with yourself and slowly work up the weights over a few cycles and time, you will get there much easier and with less to no injuries. Believe me I have been where you are.
3) I am one who believes some quad is good for off the floor, but a lot of the conventional deadlift, is hamstrings and low back for guys like us. I am 6' and pull quite a bit with my hams, hips and low back. This is also why I said to set up/get as close to the bar as possible.
For taller guys, I am not sold on GM's, I think a better exercise is the RDL done for 5-8 reps and 3-4 sets. Probably at 50% of your reg deadlifts to start. You might be surprised how #135-#225 can feel if you do them right, and how sore the hams/glutes will be the next day. Work on feeling the hams shorten while squeezing the glutes and uprighting the torso with those muscles to complete the rep.
4) As you get more advanced, you could try some deficit deads from say 1"-2" blocks (or just stand on a #45 plate maybe). You do not want to get so high that it changes your form too much. You just want a slight ROM length. I pull almost exclusively off 1-2 blocks and my power/speed off the floor has gotten much better over the last few years. If I can break it off the floor, I can most usually always get the lift.
5) GHR's are not a bad assistance either, but do not get caught up in assistance work. First and foremost put the work in on reg deads with sets and reps. I am not a big believer in not doing deads to improve.
Squatting helps my dead to and I know you squat so how ever you mix them in, that will be something you will have to figure over your cycles and experience. It seems my squat helps my dead move up, more than my dead helps my squat. Oh well.
6) If you want to build the quads some too, after your deads, a few sets of leg pressing can help and it takes the strain off the low back while still pounding the legs.
Have fun most of all and be patient, you have some good pulling potential there if you milk it cycle the work.
