Have you ever seen a Mighty Mouse slant 6?

Usually the block is OK. I understand that drag racer teams purposely leave a new block out in the elements to age for a year (cast-iron relaxes or such), then final machine it. But entire slant engines are so cheap or free, no need to fool with it.

I’ve heard the myths about finding Hemi’s buried in the ground for that reason

That's called "seasoning" a block.