Do yourself a favor and find a way to get by with a 60 or 65 liter pack. It will force you to pack light and trust me you'll be VERY happy you did. if this is a one off trip, the Osprey Atmos is a great choice at a great price. If you see yourself doing this often, then pop for a higher end back pack..

Update. I bought and used this on Mt. Rainier. It was a 5 day trip so the 80 liter was totally needed. Pack weight came at around 50lbs with water and food. It took about half a day to get the adjustments dialed in, after that it was totally comfortable. I was the only one in my club that went with the 80 liter and everyone else had tons of stuff hanging onto the outside of their packs. I only had my rope and snow picket on the outside.