How secure is it to the wall? Do you plan on putting any additional weight on top of it? In these cases I would go with the additional support. If it was just needed for looks... hmm, I'd need a better idea of what that would look like.
KA I like the idea of 2 smaller logs underneath. The mantel probably wouldn't go anywhere, but for appearance. Someone on Hometalk just posted a photo of exactly what your proposing. Love it. Even could see some deer heads hanging above it.
the bummer about it being so high is you can't see the sweet pink color of the grain on the top of the log.