Slant 6 Vibration

FWIW, the /6 engine in my '62 Dart pictured had 58k original miles when I got it in '14 and the bores were worn to the point of needing a complete rebuild. Cylinder pressures were as good as your good ones (head and gasket OK), but the prior owner had not known of a carb gas leak and had been running it for years on horribly gas-diluted oil and the cylinder wear was horrendous. It took a .040" overbore to clean it up. So you just don't always know the history of why things wear.

Absolutely pull the head. I bet you are going to find something wrong in the gasket or in one of the valves. You can also inspect and measure the bores and make a judgement on their condition. Post pix and we can see.