Wow! I feel bad for you. Can you identify a failure in the head gasket? The reason I ask, is the real chance of a crack in the head or block. The head can be magnaflux checked for cracks, but the block can't (unless it's all apart...). The lower section of the block is probably caked up pretty good with that gunk, as well. It'd probably be a good idea to pop out the freeze plugs and get in there with some strong brushes and/or scrapper tools.
An add'l (big) pain for you though...
I am no expert, I was hoping to see something obvious like a blowout. What I identified to be the problem was a an area on the rear most cylinder. The cylinder itself is discolored and the piston has more carbo. In my unprofessional opinion, the tell was the head gasket. on the front two cylinders the metal part of the gasket that mates with the surface around the cylinder was clean and shiny, the rear most was covered in soot/carbon. I think this is consistent with the symptom as well. The oil and the coolant never mixed and the coolant never seemed to be burning off, or not enough to give me the tell tail sign of white steamy "smoke" from the exhaust. When I would pressure test the coolant system, it would hold 15PSI all day but when I would run the tractor, it would start to bubble up into the overflow tank. At one point the overflow tank was clogged and the pressure has no where to go so It blew the top hose.
On that first drive, it over heated and the top hose started to expand, when I hit it with an IR temp gun, it was showing well 150-160°F at the radiator. Now I know the rad was plugged up from this head gasket BS!
Cleaning the head and block, at this point with the cost involved in recoring the radiator, I think I will drop the head off a at a machine shop and have it magnafluxed and skimmed if it's warped. When I started pulling the head I was looking at at $150 for head bolts and a head gasket. I work at a machine shop so I could easily have one of the machinist block the head up on a surface plate and indicate it for me. Now that several other gaskets, a coolant hose and a $715 radiator repair was added to the mix might as well have the head checked I am well over $1000 into it at this point. I am still rolling the dice on the block, but I suspect that is a relatively low risk.
I picked up a set of bottle brushes from the jungle website like these and plan to spend a very boring evening cleaning all the coolant passages out. What has me on edge about cleaning it up is the sodium silicate the is going to be flung in every direction, if enough of it gets in the block an into the bearings, the motor will be cooked. My plan is to use tape and plugs to block all the passages to the oiled areas of the block as best I can. Once I get as much of this crud out of the engine as possible, I will run several gallons of drain oil through the block, hoping it will grab any sodium silicate that made it past my tape and plugs. Then I will top it off with a fresh filter and Rotella. I might even dump the oil and coolant after 2-3 heat cycles.
My radiator shop told me that whatever was in there was like cement and the only option was to recore. He said that he tried to boil and ultrasonic it, once he did that it started to spring leaks. I asked him for the core back because I am curious, anyone ever see a core cake up like this? I can tell you it didn't look like that when I dropped it off. There was some buildup but nothing like this!