Whiteblaze people are very genuine in their answers. They are not going to tell you one thing to make you feel better, they are going to tell you blunt truths that they have come to learn (usually the hard way) Keep that in mind when you ask for answers and if some of those answers are hard to swallow. They really are looking out for your best interests, even if you don't want to hear them sometimes.
That being said, Not every pack is for every body. These are the steps that I would take If I were in your situation.
First I would go to REI or a local, competent, outfitter near you. I would go and get your back measured professionally (if they try to charge you money, immediately walk out, never to return) and see how "long" your back is. The representative should be trying to fit the measuring device right on your iliac crest, if they are not, ask for someone else or immediately walk out, never to return. This will get your measurements. I would highly suggest not buying anything there at that time and come back to the comfort of your own home and start online shopping or supply your measurement to the WB people here. That will help narrow down packs that simply wont fit you or are not good for you.
The more open you are about your measurements here, back and waist at the iliac crest being two very important ones, the better the people here can help narrow down your choices to better fit you.
That simple step will save you several backpack iterations and pains in certain areas with certain packs (ask me how I know this
)
YMMV HYOH