To hold either a round or square post plumb inside a post hole while you pack dirt around it, make up a couple of wedges of 2x6 about 30 in. long. Cut to length, then make an angled cut from a corner of one end to the center of the other end. Insert a wedge on each side of the post; they will adjust in the hole for either post size or hole size.
Have your chimney sweep ensure that your chimney has an appropriate liner. Chimney liners are required in new construction to separate hot heating system emissions from the structure of your home.