The recess you see are the crossbolt bosses. Some of the 406's have em and some not. But the crossbolt mains could be original who knows??? Holman Moody did build quite a few 406's for NASCAR and boat racing back in the day so it could be original. Being the 406 was the test mule for crossbolted mains and the 427 casting anything is possible. Books can't dictate what went out the back door...and lots of stuff did. Hard to say. But for free??? Dang bud good deal there!!! Keep us posted and if and when you drill bit the cyls let me know what you find. There were less than 1000 406 cars produced. So there are not many left. And your block is a keeper and very rare. At best maybe half or less of those survived.

G.