We have what seems to be a standard campus 6509 layout here.
Our buildings are connected to the Distribution 6509's and each dist 6509 is connected to both of the core 6509's
Our Internet is connected to 7206VXR and connects to the campus through a 3550 which is the source of our static routes.
That 3550 is connected to the campus LAN through a core 6509
I will attach the relevant configs and a badly drawn diagram. :)
My plan is to link that 3550 to the other core 6509 ("C" shown with tiny red line)for redundancy. We had to do some maintenance on the "A" core 6509 and took out the Internet.
I would like for that not to happen.
I was hoping that the forum could make some suggestions of ways to implement this easily.
The cloud in your diagram is awesome! What I would do is add the "red" connection to C Core like you mentioned. I would make the links between the 3550 and the two cores L3 links. That way EIGRP will see two equal routes and should load balance across them (assuming you share the load across your cores).
HTH and please rate