Well, 100+ lbs is a lot and depending on what it is and how it's loaded, could influence how you are able to carry it (upright or leaning forward). A dense item ( e.g. steel, stone) that is able to be loaded closer to your center of balance is going to cause you to lean much less than 100+ lbs of backpacking gear.
You say you're 6' tall and that your frame is an older, shorter duplex. Standard duplex frames are 26", 24" and maybe 22". I'm 5'-10" and use a 26" frame, but could use a 24".
Comfort while packing heavier loads could also be influenced by how/where you're wearing your hip belt. The "points of your pelvis" should be in the middle of your hip belt and the belt should be very tight (especially for heavier loads).
The only other thing I could suggest would be to add some pictures of yourself wearing the pack and we can provide additional comments if needed.