Following on yesterday's post about the story of the process of reason which led Brian back into full communion with Rome, I found a very good link in the comments section of his blog.
One of the biggest obstacles to SSPX's return to full communion with Rome is Vatican II's Declaration on Religious Freedom "Dignitatis Humanae."
K Gurries gave a link to his blog where he has a series of posts showing a consistancy between the Church teachings of the past and Vatican II.
On Religious Freedom (Part I)
On Religious Freedom (Part II)
On Religious Freedom (Part III)
On Religious Freedom (Part IV)