Yes, you can wrap it around the sides. I saw one on the decorating forum a long time ago, but of course can't remember who posted it. I'll go check my clippings file and see if I can find it for you. Actually there is a mantle thread on the gallery side of the decorating forum. Have you visited there?

Since heat rises, there is a concern about the clearances. There are two variables that you need to be concerned with: (1) the distance between the top edge of the firebox and where you put the mantle; (2) how far out from the wall the mentle will extend. If the distance in (1) is small, you need to keep the depth of the mantle small. But if the distance in (1) is large (i.e., if the mantle is to be placed fairly high), then it can be deeper. Of course, if you opt for a mantle made of a noncombustible material, you probably do not need to be concerned with these clearance issues.

I do not know what reasonable estimates for these clearances are. With prefab units, the manufacturer usually provides that info in their owner manual. You might want to post your question in the chat room at hearth.com. Those are reviewed by people live and breath fireplaces, and they might give you some further guidance. The last thing you want to do is install a mantle and later find out that it gets too hot. Another alternative, if you live in a subdivision of similar homes, is to ask a neighbor if you can find one who has made renovations similar to what you have in mind. Or you could ask a chimney sweep. Sometimes, to prompte good will, they'll give you some guidance on the phone. Of if your chimney is due for a cleaning, that would be a good time to ask.