Hydrolocked engine + LIM Gasket failure help?

I can't say for sure but I think your engine is due for a rebuild.

There's no fourth spark plug in the back of the engine. You have the 3.8L V6. It's either naturally aspirated (VIN code K) or supercharged (VIN code 1). To get to the back spark plugs you need to roll the engine forward. This is a lengthy process but can be done by a do it yourselfer. Once you get the spark plugs out, crank the engine, either by hand or with the ignition. Look for coolant spurting from the spark plug holes. If you have coolant spurting from the spark plug holes when cranking the engine was hydrostaticly locked. These engines are very good durable engines except for the intake manifold gaskets! They are notorious for failure. The 3.8L V6 rarely will blow a head gasket. Under severe overheating conditions the computer (PCM) will start firing alternate cylinders to help prevent engine damage. The non firing cylinders act as an air pump to force excess heat out through the exhaust system and away from the engine. I would strongly suspect the lower intake manifold gaskets in this case. If you have the non supercharged V6 replace the plastic upper intake manifold. Don't reuse the old one, they crack! If the aluminum lower intake manifold is warped replace it with a junkyard manifold or a new Delco manifold. The Dorman manifold will not fit properly! Once the job is done change the oil and filter before you start the engine! Start the engine with the new oil and filter. Let it idle for 30 minutes. Then change the oil and filter a second time. This will get the coolant out of the engine crankcase and prevent a spun bearing! While you're right there you should probably replace the valve cover gaskets as well. Your Bonneville is under factory recall for leaking valve covers. Your local GM Dealer will replace the front valve cover and gasket free of charge. If you have the supercharged V6, same deal on the repairs and recall. Hope that helps! Jim