With Old Gods installed, 867 becomes the perfect date to start, as you begin strong, and your nearest possible enemies are weakened.Personally I rename some of the provinces, Nova Rome, etc. Further Restoration and Expansion. The Duchy of Tunis is also quite close and a perfect place to move your capital, as a Duchy it is the second best in the game. As a Catholic you will be able to go on Crusade, which may provide with required territory and Piety to safely convert to Orthodoxy. When you are ready to create the Kingdom, Swear Fealty to the Basileus. An excellent starting position is as the Duke of Apulia in 1066 or anytime before the Kingdom of Sicily is formed. As an Orthodox player, fabricating claims in Latinum and Mending the Great Schism is a great way to grab the territory required from the HRE, providing the Kaiser is of the appropriate religion for you to Holy War against.
If any province in Italy breaks away from the HRE annex them using Imperial Reconquest casus belli. Imperial Reconquest can no longer be used to form the empire, only after the reformation.
Also keep an eye on the HRE and the two Seljuk kingdoms in the east. If there are still muslims on Siciliy you may want to consider attacking them at this date. Then head south and capture Apulia province. You should be able to grab parts of Croatia and entire Wallachia (just ignore the penalty since they are pagans so anyway it doesnt matter much if they dislike you). Ignore the Muslims because now they are most likely busy fighting among themselves at this date.