I think you need some engineering advice before you start on this. I dont recon to know enough about this to be sure but I believe your issue is the drought we have had, and you have clay soil. If this is correct and you start jacking up your house then we get a wet summer, you could be in more strife. All you may need is some form of rehydration. I have a cracked foundation and the house moves. I have an aggi pipe 3 feet down that when we have a dry spell the wife can empty the washing machine into, about once a month. Just thought about your extra steel stumps. Bad idea. If I am correct and your soil is causing it, you would need to dig to solid, You will never do that under a house. So you will end up with surface stumps that will go up and down like organ stops.