That smoking knocking 216 was so worn out that at some point someone had brazed shut the vent holes on the top of the valve cover, probably to direct blow bye away from the firewall. I'm 68 and remember back in the 60's that nobody pulled a motor from one of these for a rebuild, A rebuild consisted of pulling the head, grinding the valves, honing the cylinders and new rings, I suspect my motor is knocking from piston slap in cylinders that were probably never bored but just honed and rerung several times. Your main bearing description of the dowells and divots sounds like a way to keep the bearing from being spun,
