Riding with your Metcalf or any frame pack is a terrible idea. You want something on the small side that rides low. Any weight up high near your shoulders will make you top heavy on the horse. A frame is going to make moving with the horse about impossible.
On one elk hunt the outfitter recommended a lumbar pack and wanted the pack weight limited to 10 lbs. I used a Browning Billy 1500 and it worked well. He also had saddlebags on the horses so we didn't need a lot of additional pack space.
On a bear hunt with pack stock I took a rucksack. That outfitter had us hang our packs from the saddle horn on the side opposite the rifle scabbard. That worked really well.
You really should ask your outfitter what to do. I can't imagine any reason for taking a frame pack on a guided hunt with pack stock. The guides will be packing out your meat on horses or mules. That is part of what you are paying for. You should just need enough pack space for the clothes needed to stay comfortable, some water, and lunch.
Here is another piece of advice. Wear some kind of nylon spandex work out shorts or briefs especially on the long ride to and from camp. Better yet, wear them everyday you are on a horse. Also take some anti-bacterial salve for the inevitable sore spots in private places. Riding horseback in cotton underwear without undesirable consequences only works for folks that ride all the time.