I totally agree with Dave on this. For me, shoulder straps are the critical factor for fit. Torsos are adjustable or at least height-based. Belts can usually be swapped or eliminated. Shoulder straps, however, can be a nightmare. Nothing worse than having the buckle up in your armpit all day. Or rubbing your neck. Or pulling to the sides all day.
I've only had ONE pack that ever worked for me stock. That's the GG Nimbus packs that are long, narrow, and pivot so they always seem to find the right place to sit. I've modded at least a dozen packs to try to get similar setups. I've only been able to get one sewn-on set of shoulder straps to sit comfortably. It always amazes me to see big guys willing to wear the UL packs with horrible shoulder strap fit. Makes me wonder if they've ever had a well-fitting pack. It's like getting great footwear for the first the time - you'll never settle again after you find it! Heck, I'm not even that huge anymore - a regular XL in tops - and I find pretty much every UL pack I've tried to be weak in this area.
Currently using a severely modded GG Crown (2lb with pivot straps), a cut-frame GG Nimbus Meridian (2.5lb but insanely comfortable) and a DIY pack (1.5lb with GG straps). Would love to find a 1lb comfortable pack to haul my typical 11lb load, but I haven't found one yet that doesn't need the shoulder straps rebuilt.