Barns in Maine such as my fathers dated 1838 still stand straight and solid. The barns were built on rock posts and stone foundations using hemlock, white pine some with bark left on, tamarack aka eastern larch, and what ever was available. Secret to the success is dryness. If a roof fails in new england these old barns will fail in a little as three years when the water hits the old dry wood and rot starts. If you're putting posts in the ground, use PT, if you have termites take local recommended precautions, and put on a good roof structure with high grade steel roofing that sheds water well away from the wood work. Aught to last as long as you need the barn. Keep the roof tight, the foundation area dry and post a picture in 165 years and show us how you made out.