How sketchy are the stock valves?

Since I got my car I have spent a lot of free time freeing up the engine that sat for over 15 years.

My plan was to get it running and then slowly freshen up the car one system at a time instead of doing the complete teardown and not driving the car for a year or more.

I got it moving, then I had to deal with some stuck valves. Then I did a compression test with some cylinders being great and others being awful.

I did the rope in the cylinder trick, removed the springs, and spun the intake valves back and forth in their seats with a right angle drill.

When I was done, all but two of the cylinders were 155-165 psi on compression (dry), which I thought was pretty impressive under the circumstances. I was attempting a do-over on one of the low cylinders when I dropped one of the valve locks into the nether regions of my engine compartment (not the engine thankfully). I can't find it. I have run a magnet everywhere and blown compressed air all around as well. It must have fallen into a frame rail, be sitting in part of the engine mount, or I inadvertently kicked it across the garage. I even dropped the other half on purpose a bunch of times to try and see where it might have gone.

In searching for a replacement valve lock (located one that I can pick up in the morning) I discovered many tales of woe involving the stock valves.

My dad has 2 spare Cleveland blocks in his garage that came out of Mustangs headed for the crusher so I was willing to stick to my plan above and just run what I have until I got fairly strong evidence this morning that this is probably the original block. I don't have either motor or body tag, but apparently this car and others made around the same time never had them. My engine is stamped with a number that is 5 away from the number on a car that is 6 away from my VIN.

If I am keeping it under 6000 rpm and just driving back and forth to cars and coffee type stuff am I risking catastrophic block damage with those valves?

If it matters, I think I have closed chamber heads - 4 on the corner with the dot but the date code shows they were made in December of 1970.
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