I suspect that you might need an XL. At 5'6" and 164 lb with a 42"+ chest, I need the L. As mentioned, it fits like a fleece jacket and I can get 2-3 light to medium layers under the jacket. Again, I suggest that you discuss this with FF customer service.
I hope that they have the Quantum in stock for you. Also, not only is the system light, flexible, and relatively inexpensive, it is extremely well made and designed, and as I indicated to extremely high workmanship levels. I think you will be happy with your purchase.
I wish more people on this site were a bit more open to a modified more traditional sleeping bag system (with down that can be moved inside the baffles) than using the very expensive and well made Arc sleeping bag system.