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I'm sure you probably looked into it already but I've heard a similar clicking sound from a cracked exhaust manifold. There are some YouTube videos where you can hear the sound and that demonstrate an easy fix with furnace cement. Good luck!
Never heard of anyone making a positive diagnosis of a wrist pin with a stethoscope.
If you've eliminated the flexplate as the culprit and in fact are hearing a sharp knock you can waste time trying to determine which cylinder it is or if it's mains but regardless the engine needs to be rebuilt or replaced. Where do you live that $50/hour is the labor charge? If it's that cheap a new engine won't cost you much to have installed.
1)The engine is actually 39 years old, not 40. 2) Is the engine otherwise smoking and/or using oil and/or using coolant?
Do you have AAA?