Possible head/valve problem

[Hammerhead]

That's not a very accurate way to measure clearance.
If the head is original with that many miles, it's time to measure them properly and be prepared to replace the head.
Use a dial gague, they don't work well with the standard radiused needle end, so see if your supplier has a "lever" to attach to it for more accurate measurements. The radiused ends don't sit well, as there are no smooth surfaces on the rockers, and the they rock, not straight line, so they don't stay uniform.
I don't have a pic with me, but I have a small "bar" with a small hole on one end. I unthread the radiused tip and the threaded part of the tip goes through the small hole and tightens back up. This makes a nice large surface that isn't affected by the lack of smooth surfaces and measure the stroke well.
I hope I explained that well enough to get an idea of what I'm talking about.