68PK21 440.6bbl
Old Man with a Hat
Without having to read through all the paff again I think your problem originated with a frozen/stuck closed heat riser valve on the exhaust manifold. I seen quite a few clogged up heat riser passages clogged from the lack of using this.This is bolt 17, from the very back of the head, up top. Note the apparent lack of any lubricant on the threads. This one appeared to be put in DRY! Hmmm. It sure as **** required more than 50 ft-lbs torque to break it loose. BUT after the outer 4 bolts, 2 in back, and 2 up front, THEY GET ABSURDLY LOOSE!!!! NO WONDER THINGS ARE GETTING HOT! The inner bolts came loose with VERY MINIMAL EFFORT! Most made a very non-committal, quiet click on the 50-ft# setting, but nearly half didn't even do that. These bolts had loosened over time.
Basically when the valve is frozen shut and constantly driven in hot weather it will cause a hot spot in the head and intake manifold at the crossover junction of head to manifold. Overheat & cool over many months/years lead to the bolts on the intake manifold loosing up and oil getting sucked into the crossover passage carboning it up from passenger side to drivers side. Heck I've come across one that was carboned up solid on both sides and even the heat riser crossover valley was clogged requiring some hammer and chiseling with a long sharpened rod to free up the passage. Your lucky as it looks as if some oil got into the water passage in the head/block area causing the clogged passage and it didn't go the other way (coolant into oil)
I don't know what you were going on about with the engine block(?) heater, I really don't have time right now to sift through all the threads but the 'Evil' heater is not the problem IMHO. If it's a factory freeze/core plug heater and used correctly it wouldn't cause any problems I'll bet.
(edit) also if you are going to re-use the heads & intake manifold I'd check the intake & head surfaces with a straight bar for flatness.
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