Having built a log house a few years ago I was faced with the same concern. An experienced log builder advised me to cut the vertical members 1 inch shorter than needed, then use four 1/4 pieces of plywood cut into small pieces as shims. As the house shrinks take out the plywood as needed (if you want to take 1/2 inch remove two pieces). I found it easy to lift the vertical members using a hydraulic jack. Hope this works, it did for me.