Leleling the floors is considerably different from raising the house. Tool Guy and Guyod have the correct answers, just combine them and go for it. As long as we are just leveling the floors, I don't think there would be any plaster repair needed. Floor leveling usually happens at mid span of the floor joists. Just use a 4 X 6 beam, the jacks and some concrete blocks to hold it when it is level. Under the house we don't have to worry about freeze depth; simply dig down to undisturbed earth, place a concrete block on its side (for more bearing on the dirt) then stand the blocks up to make up the rest of the distance. I would not use any wood unless it is treated and rated for ground contact.