Sounds like you have DT's Squat 900 article covered!
The first thing that jumps out is that you are using a wide-stance pl-style technique using a pretty narrow stance...Have you tried working your stance out another 6-8 inches? If you aren't cursing the makers of normal squat racks, you probably aren't wide enough.
When you fail 1" above parallel, are you falling forward, losing your arch, or just stalling out?
If falling forward (the most common mode), GMs would seem to be in order. Possibly someone to give you a few verbal cues ("Big Chest") to help hold you on the course...SSB/Cambered bar squats would also help.
Just stalling out might be hamstrings, but might also be stance...Nobody has strong enough hams, myself included! Widening the stance some will put your hips closer to the bar in the hole and givve you a bit more leverage...
You might also try some gear...Get a cheap suit that will get you over the stick, but still let you go full range. Work up until 600 feels like a warmup weight and then try it without the suit...Shirted benching has carried over fairly well to my raw bench...
Last: Find someone else, teach them to squat, and have them watch you.
Honest, it'll help.