Add in the XFN magic by replacing your <head> tag with <head profile="http://gmpg.org/xfn/11">
Add <link rel="me" type="text/html" href="http://www.google.com/profiles/your.username"/> anywhere inside the head tag. (Of course replacing the href with whatever your profile URL is.) After <link rel="alternate" type="application/rss+xml" title="RSS" href="{RSS}"/> is a good place.
Go to Google Buzz in Gmail and click on “N connected sites” and there should be the option to add Tumblr. (It may take some time for it to show up, but be patient.)