Core Plug Replacement   As I posted on another thread, the core plugs on the left side of the block on my engine had all corroded to the point where something needed to be done.  Here's what they looked like:     These plugs are just under the intake and exhaust manifolds, and can't be reached with the manifolds in place.  At first, I considered removing the manifolds to get at them.  But then I discovered that there's another plug in the rear, right up against the