Interestingly the inside of my head casting is nothing like the diagrams shown here. I took the bottom coolant plate cover off to replace a broken coolant hose and looked up in there with a flashlight and mirror. The entire front of the casting is nearly solid iron surrounding the spindle cartridge not a empty box as the diagrams show. It goes all the way up to the pulley so the cartridge in completely encased in cast iron. I'll snap pics at some point but right now I'm pretty solid on keeping the existing casting and just boring it out to 90mm.