Ignore this if you're repping 600+ on the deadlift, as it may be something else.
I started having problems one day earlier in the Winter after squatting with headaches. It happened on a 20 repper when I wasn't breathing out, trying to just finish the set as fast as I could. Then after that, any time I worked out I would get an absolutely debilitating headache, like I was out of commission for a good 2 minutes trying to get my s$#* together. I thought I had a brain aneurysm or something. I tried to focus on the breathing, but that wouldn't help afterwards.
I never did find out the problem, but I decided to try out wearing a hat during my workouts (like a beanie or a hoodie with the hood up), thinking it was definitely related to blood pressure and that the extra warmth around my head would dialate my blood vessels, relieving some of the pressure. Now, I don't know if that's what happened, but me doing that saved my life and my workouts. Never had a headache problem after so long as I remembered my beanie. A month or 2 later, I stopped wearing one, and I noticed the problem was gone. Maybe try that out now? I realize you only have problems during the deadlift, but it could still help. As for breathing, definitely keep the air in until the top, and I recommend exhaling only enough to relieve pressure but not enough that you lose your form. I've seen some dudes squat 800 at my gym without letting much air out the gauge and not have headache problems, but then one day they experienced what I did during a deload. Go figure