Opinions sought on odd engine knock

52 years old would be 1962. Yours doesn't have hydraulic lifters to "pump up", and I understand there aren't even oil passagess to your solid lifter bores. There is a gap at the valve tips to rockers, but that should make a slight ticking sound all the time.

I have read that a worn main bearing is a deep sound (never heard). You can check/replace the main bearings and rod bearings from below after removing the oil pan (I need to jack my engine up slightly). If so, also replace the rear main seal (push rubber half around crank). Check compression first and do a complete rebuild if <100 psi in all cylinders. At a minimum, turn the engine over 2 times by tugging on the fan belt and see if you fight 3 tight "air springs" each rev and hear them hiss down. If so, your compression is great regardless of what your gage test says.

Most likely it is something more benign like "piston slap" or timing chain slop.