You said you inspected the adjuster screws, but did you look at the valve tips? High mileage valves can be cupped by the scraping action of the adjusters. End of the valve you can see with the cover off, not the big end. This will make the valve end cupped, giving you a false reading with the feeler gauge. On an engine with this condition you need to use a dial indicator to measure the true valve lash. just bolt the indicator to a solid place on the head and make sure the plunger is directly in line with valve you are measuring. I know it is a pain in the rear compared to the feeler gauge, but if you do it right, you will adjust the valve lash VERY precisely. good luck!
