Yours must have been about the last year with solid lifters. They changed to hydraulic lifters soon after. People say well-adjusted valves in a slant should sound like a sewing machine. Perhaps that is with the valve cover off since I don't recall hearing anything special in the decades I owned my 1969 slant and I did adjust valves regularly, plus mechanics did a few times. A loud clacking sound could be a worn camshaft and lifter bottom. But, I suspect your 1980 has the "peanut" head without the removable spark plug tubes. In those, you must remove the head to remove a lifter to inspect. I bought an $18 USB borescope camera (ebay). Perhaps that could be sent up the oil plug hole to view the bottom of the camshaft on your next oil change. It works great to view tonsils and has a built-in LED light. Check Google Images of slant engines to see if you can even see the cam lobes from below. Some borescopes let you adjust the head tilt w/ knobs.
