I picked up a used HMG 3400 Ice Pack recently. Pretty heavily used, but still quite functional (after a bit of extra seam sealing via Aquaseal).
According to HMG, I should be using a size large pack, and that's what I have. As long as I am carrying loads up to 25 pounds, and I distribute the load correctly, the pack carries very nicely. Above 25 pounds, however, I cannot keep the pack from slouching back to the point where it applies enough pressure to my shoulders that I get sore trapezius muscles after a while. I really (REALLY) hate sore shoulders at the end of a long day. So, I added load lifter straps to the pack and extended the frame up an additional 4" to accomodate the LL straps. I have had one 10 mile day hike with the pack carrying a light load over mountainous terrain, and one 11 mile walk on easy terrain carrying 35 pounds since doing the mod. I can't lean my head all the way back without it hitting the cross bar of my new frame, but I might do a little bending to fix that. Otherwise it made a noticeable difference in my ability to hike with the heavier load and relieve most of the weight on top of my shoulders.
I canibalized the inside pocket for material since it was in the way of the mod anyway. I sewed the top and either side against the body of the pack and added velcro straps that attach to the stock aluminum stay tensioner straps:
The new frame is made from 58" of solid 1/4" aluminum rod ($1.42 at the local welding shop) bent with a wide flare at the top to support the load lifter attachment points, and then neck back down to the width of the stock stay sleeves. Here it is outside the pack:
This gives you an idea what the resulting frame height and LL angle is:
