I had a hunch it was the bigger size. (hadn't measured it yet) I did find this website page that seems to sell it:

http://www.truckandcarshop.com/trucks47_59/t47_058.html

They have a 10-spline and a 26-spline in the 11" size. Hopefully that does it.

If that doesn't do it, I'll have to see about resurfacing it. They looks rather rusty, I'm not sure I want to use the same clutch parts.

I'm assuming the throw-out bearing on that same web page will work just fine?