Know what part of the trail you will be doing might help, but, regardless, here's what I'd do differently:
-I would take shorts and no long pants; maybe extra shorts so you can do wash.
-I would want a rain jacket, poncho, or at least an umbrella; that Ghost Whisperer is not really a rain jacket. I guess you could get by with it if you're in the southeast.
-I would plan on being in a short sleeved light wool or poly shirt; the long sleeve shirt will likely be a night shirt.
-you list spare socks but not a first pair. I really think there is no substitute for 2 pairs of light wool socks.
-I carry only a little more than half the amount of food you estimate
-take the AWOL Guide
-include money, license, etc.
-I would never carry anywhere near 64 ounces of water on the AT. Look at your AWOL for water sources. I typically fill up with 16 ounces.
-I think you've underestimated the weight of your extra clothing.
-I think that bivy is going to be a sweat bag; I would leave that at home. You could stay in shelters only and get by with just a head net or breathable bivy; add a light tarp if you want to spend time outside of the shelters
-Add some toiletries like soap, toothbrush.....
With regard to the pack, I carry a zero a lot of the time and like it a lot. Once you get over about 20 pounds total weight, it starts getting a little uncomfortable; over 25 and its not great to carry, but doable for awhile. A young, fit person can usually be comfortable with a little more weight than an older person. SO I think you need to dial in your total pack weight a little before you make that decision. I can see some weight you are going to need to add in your list, but I think you are likely carrying more food and water than you will on most of your trip.
Hope some of this is helpful.