According to stone glacier the large starts at 37. If you want to try it I’d send it to you. I would just ask that you pay the shipping both ways if you return it.
I'm size 35-36 myself. I originally purchased a pack with medium belt and it did not cover my hips. I called SG and they told me for sure I should be in a large. Purchased the large and it was much better. Just did a 12 mile trip with 80lbs and the large belt worked great. No matter how tight I made the belt, I still had more room to tighten more.