Evaluating engine lower half on 1950s engine with ~600 hrs and 40 yrs SMOH

Take a one time oil sample.  Take the sample out of oil reservoir.  Be sure to run the engine up and take a warm oil sample.

As side note one of my airplanes has 1300 smoh and 51 years of time.  The aircraft has been stored in it closed hanger in the Arizona desert.  It just had regular oil samples and has been in the family the entire time.  So a little different situation, because I know the history and who's been flying it.
 

Thank you forthe advice. Pulling jugs will require a serious discussion...how much risk does the buyer agree to take versus how much does seller want to sell, and how can this be fairly priced into the sale.
Immediate oil analysis sounds like a good data point if the oil has run long enough in the engine. Maybe a good intermediate indicator.
Thanks all!

It's possible to take an oil analysis sample without draining the oil. It might be worth a one time sample to check if there are any values way out of line. For example Blackstone's charge is $35/sample (but their processing time takes a while).

where the plane is has some impact as to the condition of the engine. If it was in a humid area, I would worry about rust, but if it was in say the southwest, I would not be too concerned about it. The only way to really see the cam and lifter faces is to remove 2 cylinders on the same side. You might be able to see some with a borescope if you drop the oil tank down, and go inside the case there.

You hit the nail on the head about the concern at the price of an overhaul versus the value of these planes. That's where the educated assessment of engine condition is so important. Thanks again.