Back positioning in front and box squats

A couple quick questions about my back for front and box squats:

  1. Front: when I go atg, the last 6 inches cause my fairly upright stance to suffer. No matter what the weight, my back looks rounded at the bottom, but if I straighten up anymore I feel off balance. I should let you know I’m about 6’3, but my legs aren’t freakishly long - everything is basically in normal proportion. My knees go over my toes a bit, but not much. I feel fine when squatting, but my friend noticed my form yesterday, so I thought I’d ask around. Will this improve as strength increases, or does it not matter, or should I cut out the atg (say it ain’t so!)?
  2. Box: when I sit down, my back has a tendency to come upright (since it’s bent over more during descent for balance). So before I stand back up, I have to lower my back again. Should I keep my upper body position static when I’m sitting on the bench, or just do what’s comfortable, even if that means some movement?
    Thanks guys!