It might be tough to get stock blanks from 18 in diameter. I'm assuming that measurement included bark. So, after removing bark and sapwood, maybe 15 in. Then, cutting around the heart, would need to offset a couple inches for plainsawn. With a 3 in slab, the curvature might have the shortest flat of the slab around 4-5 in. For quartersawn, maybe 6 in x3 in. Then add (or subtract in this case) shrinkage during drying.
Can the older, wiser guys say if I'm on the wrong thought?