RBT, I have used that same tire in a 26" size on a build, Hawth Rod, in the past. I got by with using a dremel and just taking a few layers off the chain stay / seat stay to make it fit. And yes, spot on trued rims are absolutely necessary!
There is a lot of rubber on that protruding edge, and I would not be afraid of grinding some of that off either. Take it slow, a little bit at a time.
What psi are you running in the tires? I have found that even 5 - 10 psi less than max will allow the tire to fit in tight spaces, and provide a better, cushier ride quality. So, a 40 psi tire pumped up to say 32 or 34 psi might help your situation as well.