Our friends at OneProjectCloser.com did a nice job with their hardwood flooring article.  Ethan and Fred run into issues like all of us do when we build. Their solutions provide an alternative you may not have considered, as an example using roofing shingles as a means of leveling substrate.  Check their article out for this and many other good ideas on installing hardwood flooring, http://www.oneprojectcloser.com/how-to-install-hardwood-flooring/.
