I wouldn't hesitate on a rib shot with a good bullet from a .243. I've seen the 100-grain Nosler Partition go through both shoulders and the spine of a big whitetail--and exit. That's a lot more bone than a moose rib.

That said, the 6.5x55 is definitely a moose cartridge.

“Montana seems to me to be what a small boy would think Texas is like from hearing Texans.”
John Steinbeck