Ken,
Little late to this party, but I'd be real surprised if the moisture content of these planks is as low as 15%. On the stump a tree's about 80% water by weight. Air dried material will usually get to about 20%. Unless this stuff is heavy or fuzzes when ripped I wouldn't worry. If it is/does rip it and stack it in a dry well ventilated space with stickers while you build your forms. In a couple of weeks it should be dry enough. The Sharkers always said to allow a year of air drying for each inch of thickness. I've noticed that a 2 by 6 left to dry for a year is dry about a half inch in from each side. So it seems the Shakers knew what they were taking about. So if this stuff was sitting around the lumber yard for a few months you're at least half way home.