1. Does anyone have an idea what this black stuff is? Carbon? Stop leak? Copper erosion from a radiator? I hope it's not crystallized oil which would mean
dissolved passivied silt is what i call it. yeah i found this in all of my engines when i was told "just flush the block and youll be ok" NOT! first off to get the block to drain you need to remove the drain valve on the side of the block. then stick a 1/8" welding rod in there and if you have to pound it in go right ahead. while working the rod deeper and around it should start to weep. when it does its yours to take but it just may take some time. since you pulled the soft plugs and its on a cradle i would pull the head and once you get it off youll know why i said to do so.
2. From the coolant drain hole to the water jacket, is there a direct path or does it turn left then go up? This would help me a lot to clean out the passage.
like tim says the hole in the front from the water pump makes easy access. with the head off and the water pump out and you already took the soft plugs out youll be able to see whats in there and the progress you make removing the crap which there is more in there then you can fathom. start by getting a clear path from the drain plug all the way around #six cylinder and to the block wall on the side where the starter is for the chunks to flow out which youll most likely have to bust them up to fit out the hole. then carefully working the cooling passages in the block from the top down to the drain plug then work yourself systematically to the front of the block. careful not to make a mess and get it in the cylinders. i always put the piston up where i was working to keep any water from ending up where its not supposed to be. if you do take the head off make sure to rod out and blow out the cooling passages in the bottom side of the head. i made a gadget out of 1/8"id copper tubbing to fit on the end of a garden hose with a shutoff to use for flushing and flow conrol. after i did all this which was in preparation of a engine build, i filled the block up with XXX boiler cleaner and left it set for a week. i thought i had it cleaned from rodding HA! all i did was remove the top surface and chunks so that the boiler cleaner could get to the block and do its thing. flush many times. since the engine is where it is its a good time to do this.
with a clean block and a recored radiator the only time it goes above 190 is when im hauling a load home on a hot summer day with 4-5 tons of stone on the back of the dump with a 59-235 on a hill called the "blue mountain"
