That's a perfect example of what I'm talking about.
I've done this with plenty of old cars and all they hype about Seafoam and all the additives people buy into and it usually causes more harm than good.
Be careful when getting after one that has been neglected.
People want to start cleaning, flushing, sterilizing, and they end up causing chunks of stuff to come loose and plug up orifices, injectors, oil galleys, fuel filters.
Do NOT run seafoam or similar products through the gas tank or oil...
Run a Rotella diesel oil and change it every 3000 miles to help clear up most of the junk inside the engine. Run a screwdriver through that hole to clean it out, install a new PCV valve and hose and drive the hell out of it!