You seem to know this and it is implied by other posts but to be clear, the limbs are not suitable for lumber, they will warp because the stresses were unequal while it was growing. The limbs will make good firewood. The straight sections of trunk will make the most stable but not terribly interesting lumber. Crotches will give you highly figured material that will not be very stable. The butt including what is a little below ground is highly valued gunstock material. The part at ground level or a little below is figured but not very stable. It goes into the butt stock. The material a bit above that is where the action of the rifle goes since it is straight and more stable. Getting the butt out of the ground will not be easy, however, and it wouldn't be surprising to hit rocks trying to cut it up. So it is often not used. If it was me, I would want it cut down with at least 18 inches sticking up. I would then dig out the big roots and cut them off with a chain saw to free the butt. It would be a lot of work but I think the material would be worth it. If you clean things up with the chainsaw, there would be less risk of damaging a bandsaw mills blade.