It sounds like you know that your lower back should be neutral, not overextended, so do as much chest up and upper back tightening as you can without overextending the lumbar, and squeeze the bar off the floor.
Just don't rush past the chest up step. If you've seen Alan's deadlift videos, you see a distinct step when he squeezes his chest up (and starts to grimace). Don't skip that step, even if you think it isn't working for you.
If you've not seen it yet, this is a good article on thoracic spine extension/flexion. While it doesn't deal with kyphosis, you can still read some more cues that might help. Identifying and Correcting Thoracic Spinal Flexion in the Squat | Bill Hannon
Again, I'm not an SSC, so the whole 'blind-leading-the-blind' thing applies here. The search function reveals some good comments from Andy Baker a few years ago (Thread title "dealing with a hunched back").