I have no personal experience in shooting either moose or brown bear with a bow and arrow. Fred Eichler has shot most of the grand slam of North American game with his Palmer 54# recurve, including brown bear and a large Alaska-Yukon moose. So, if you are using an efficient 55# bow with the arrow weight and broadheads you mention, have everything well tuned, and hit the animals in the proper spot, you should have no trouble getting good penetration. Fred's moose was a long ways off when he shot, but his arrow was well placed and still penetrated well. I am not sure what kind of longbow you have, but not every bow is terribly efficient. Ed Ashby had about an 83# longbow that shot the same arrow at the same velocity as a more efficient 65# bow. So, you can't go strictly by draw weight. Try shooting your arrows through a chronograph and make sure your bow is getting good velocities. A fast 55# bow will be enough. A slow 60# bow may not be. If you can shoot a 63# bow better than a 55# bow, by all means use the heavier bow, assuming they are both equally efficient.
Allan