There could be all sorts of nasty stuff going on with this engine. Bad head (most logical due to oil contamination and the suspect damp spark plug), crack in the block somewhere (not likely but you never know...) Rear freeze plug is probably rotten. Are you getting pressurisation like from the freeze plug anywhere else in the engine like from the radiator cap? I would doubt the exhaust gas pressure would be enough to blow out a freeze plug but I think this engine is toast. Only way to know is pull it down and take peek at it's innards. If you know anything about the 4G64 the one thing that seems to consistently die is the ECM. Usually bad capacitors on the circuit board. Easy enough to remedy as long as the board hasn't been damaged by the caps leaking. Would you even able to legally swap the injection onto your G63B in CA? If you can find a running 4G64 and you can get this truck cheap enough, it might be worth it as a side project.