Somtimes,, the obvious things are overlooked.. Since you said it was running fine and then starting overheating because of grass in intake,, I'd tend to think that the running hot of the engine was a great indicator of the problem.. 7 and 8 cylinder prob has water in um mixed with unburned gas..Oldsmobiles don't like "no oil or hot oil" situations. Spark plugs and carb rebuilds etc are preventive maint. and won't fix overheated or abused engines.  An engine overheat and oil overheat lights are cheap insurance against these delimas.
     Sorry bout your bad luck.  But on the bright side,, you will have a new engine.