So, I've got the valve covers off (440) & just adjusted the valve lash. I notice that there is some coolant "weeping" out of the rear/lower valve cover bolt hole on the passenger side. Is that a cracked head?

I'm "hoping" those threads just run really, really close to the head cooling jacket & I tapped too deep cleaning up the threads.....but I'm just "hoping".

Original cast iron heads are well known for cracking. Not a big surprise, they are 50 years old and have had lots of heat cycles by now. If it is a very small crack you might be able to seal it up with a stop leak type of product.

If I'm not mistaken the lower end holes go into the water jacket that's why Ma Mopar put studs in the holes

The valve cover bolt holes don't (normally), at least on any iron big block head I've ever had. Only the exhaust port holes...

It's not unknown for valve cover boltholes to hit water. Not supposed to but stuff happens.

I had on with a broken vc bolt
when I drilled it out went too far hit water
didn't leak w bolt or stud in it

If it is leaking fix it now, it won't fix itself
You might want to use some two part epoxy, JB weld or?, on a stud for that hole, drain the radiator first and clean that hole out thoroughly first though before starting to patch it up

Thanks everyone. I could not see any visible cracks & the engine has been running strong. I put some RTV silicone in that hole before screwing in the valve cover bolt....we'll see. No sign of antifreeze in the oil.

I had a 906 head that one of the bottom corner valve cover bolts went into a water jacket. A little sealer on that bolt and no problems.