You try talking to Kifaru about that. They might be able to help you with a modified pack suspension. E
I spoke with Patrick a few years ago when I had this problem with a G2 Longhunter with a medium belt. I said I think I needed to swap out my belt for a small size since I've got a 34" waist, but he talked me out of it. I never did get rid of the slippage issue, so I passed the 30 day return window and had to buy a small belt. That helped some but didn't solve the slippage.
The Timberline 1 I used on my elk hunt this past year had a small belt and did the same thing with heavy loads.
The 9 Kifaru packs I've had feel GREAT with normal loads (up to 50 for 24" stays, up to 70ish for 26" stays) but they flat don't work for me with heavy loads.
My experience with K packs has been good, but for someone buying a new one I would recommend you load it with 50 lbs and go for a 3-5 mile hike, then load it with 100 and do the same. Do that before the return window passes. I'd recommend that for ANY hunting pack BTW.
I've recently picked up a DD Shortbed and an unused DD T1 from craigslist and am putting them through their paces with 100 lbs of sandbags. So far so bad....the lumbar pads on the DD belts slip as well.
I also traded for a DD Stillwater (internal, same suspension as Terraplane) and so far love it with regular loads. I haven't tested it with 100 lbs yet, but am afraid it will slip too.
I have more hope for the full wrap belt on a Kelty Cache Hauler than the DD frame packs. I borrowed a Kelty and will test it out this week.
