Ok. the Cervelo site itself helped out some -
http://www.cervelo.com/bikes/SLTeam.html#Geometry - you can see how they measure the top tube and how it comes out to 58 cm.
I would put your seat back to it's center position and reverse the stem. Then re-measure the top tube and reach - it should get you close to the 70.55. From there, I'd ride it for awhile to get the feel. Then, down the road, look into changing the stem if you feel like it.
Beautiful bike - I love the philosophy of the company.
55/Rad