Blow a head gasket or cracked something like a head or block. Maybe even shattered/cracked a cylinder bore. Combustion pressure guarantees that the cooling system has 15+ PSI in it. Combustion pressure and gas eventually air locks the cooling system resulting in running hot. After all the combustion pressure forces a lot of coolant out the overflow. The leak can be slow enough that the engine will run vaporizing the leaked coolant or breaking and bending stuff to keep running past attempted hydro-lock. When the engine shuts off the 15+ PSI coolant is forced back through the cracks, blown gaskets, etc. After all it has lots of time to fill things up when shut off.
You rotated the engine with this much coolant in it. IMO you bent rods and appear to have lifted the passenger side head enough to shoot out the coolant
. Broken or stretched head bolts? Warped head?
Was this engine ever rebuilt in the past? Maybe they reused the disposable single use TTY head bolts.
Find the failure and this will include pulling the entire engine so you can check the cylinders for cracks esp from GM's erroneous tool chip stress riser from drilling the piston oil squirts. A head gasket failure in this case is NOT a clear single failure! The head gasket could have failed from a crack's resulting coolant leak attempting hydro-lock that stretched or busted head bolts and left you with a clear head gasket failure
Symptom. 2nd reason to pull the engine is to re-ring it as the overheat likely ruined the ring tension and it will have more blowby when you get it to run again. IDI diesels, especially the 6.2/6.5, ruin rings when they get into their very low limit of hot. Best reason to pull the entire engine is it is so much easier to work on pulling it out and back in complete with exhaust manifolds and most accessories.
It's a lot of work to just change the heads and have to re-do it because you missed the cause. Freshening up the engine after it's given a crack free bill of health, frankly rare for a 6.5, will result in reset of it's life span sometimes with no more than rings and gaskets. If it takes more than that its' time to look for something else depending on your budget from low mile military take outs to re-ring or all out P400 6.5 replacements.
