Conditions were similar on the Saturday I rode CFC, and I wound up borrowing someone's knee warmers because I'd neglected to bring my own. That made all the difference in the world - I was freezing without them, reasonably comfortable with them (or as comfortable as one can be on a hellishly difficult ride in cold rain). On my stubby legs, "knee warmers" pretty much cover everything. I'd wear the full length tights, myself, but I'm the kind of person who would much rather be too warm than too cold.

To me, the kicker is that you anticipate having to stop periodically. Chill sets in very quickly in those conditions (I'm reminded of the shivering teeth-chattering masses at Boston, and it was sunny and mid-50s that day.) Myself, I don't think I'd need the cap while riding, but I would very much want it any time I stopped.

I would even make sure you have a few reflective emergency blankets in your kit, in case someone winds up having to wait for the sag wagon.