I would recommend an external frame pack for 50+ lbs. A good hipbelt is important as this is where you are trying to transfer the weight. I would even suggest an "L" shaped external. This will provide a shelf which will act as a lever making the fulcrum right at the hipbelt. One thing to realize is the packing of an e-frame is different, however it is more forgiving than a poorly packed internal frame pack. Even so, avoid hanging stuff off of it. Get it inside, or lashed tightly (on top is preferred. while I would also recommend trying to avoid carrying 50+ lbs, there are specific reasons why one might need to. Enjoy. Be safe, a heavy pack increases risk of injury.