I just tried to replace mine yesterday, the nut came off but the by broke off near the gasket. Did you ever get the bolt out? I am going to try to get it with an easy out but not sure if it will work. i will just loosen the cooler and replace with one bolt if I'm unable.
Trust me, you don't want to use an EZ out on this. It will just break off inside and then you are really screwed. Either weld a nut on it which will get it out or just drill a new hole in the same spot the best you can and tap it.