You're not having trouble keeping your heels on the floor because of your head position. You're having trouble keeping your heels on the floor because you are performing a high-bar squat with the bar in the low-bar position. Note the excessive and continuous knee travel. Re-read the section on knee travel in SS:BBT. Knees should be set in place after the first 1/3-1/2 of the ROM of the squat. Yours never get set - they are in motion continuously.
Once you set your knees, sit back, and bend over.