I love my deuter act lite(internal frame), and it was the most comfortable with 35 lbs of all the packs listed so far. I literally spent all day trying on every pack REI carries.
That being said, I'm not sure that I would look forward to carrying 45lbs in it all day.
I recently picked up an old Jansport external frame, and after removing the top cross bar, and the "shelf" bar, both unnecessary. It came in at 3lbs 5oz. Lighter than my Deuter, and feels like it would carry the load better. I'm going to use it on my next trip to find out. At the very least it will get rid of my sweaty back!
Oh, and it only cost me $20!
You might be able to cut even more weight if you get rid of the pack bag and just strap stuff sacks to the frame. Be carefull that all the stuff sacks and straps don't add up to more than the original pack bag.