Though I don't know if this is timely enough to help you make a purchase decision, it is still possible to find under 100K mile engines from C bodies. For some reason many of them didn't get driven much.
Seeing that you come from a cold climate, check for any rusting or discoloration from seepage around the casting plug areas that would indicate block cracks from freezing. If it always had antifreeze in it, it should be OK.
You must factor in the cost (minimal) of changing the timing chain assembly, valve seals, valve cover and pan gaskets, and pick-up screen as 43 years will have pretty much disintegrated the seals, nylon timing gear coating, and gaskets regardless of mileage, and deposited all the chunks in the pan and screen.
If you can drop the pan, and it's a stock original engine, you will find a ton of horrific looking junk in the pan and screen, as well as a thick coating of sludge on the bottom of the pan.
The sludge will have a metallic sheen to it if you scrape some off with your finger, but all of this is quite normal as long as there is no big chunks of metal from bearings (that you can actually pick up with your fingers). All this can be a big bargaining chip while negotiating price.
If none of the above is present, it means that someone has had it apart at some time or another, which may or may not be a good thing depending on why it was disassembled, such as timing chain replacement (good), or some other problem (good or bad).
with the pan off you will also be able to see if there is any rust in the cylinders. Light discoloration is fine, but heavy rust is definitely bad.
Hope any of this helps.
