Leave the bark on for a while and let it set on the ground if you want the outside to have a worm eaten look after you remove the bark. You say you're going to slit it. Do you mean that literaly or are you going to saw it? I like my mantals sawn 3 sides and live edge on the front. With size log I would shoot for 14" x 6" and get some side wood out of it. You could also get 2 half rounds, actually more like 1/3 rounds. Cedar is naturally pretty dry. Should be dry enough for your purposes in about two months.