340 coolant trouble

I'm not entirely sure about compatibility between the old engines and the newer stuff (bolt holes, etc.) but I'm wondering if the pump is a reverse rotation piece for a serpentine drive setup. I've seen it happen as a result of parts store clerks grabbing the wrong part. It makes a person crazy. However, this wouldn't explain how it didn't kick into 'puke coolant' mode in the shop.

As someone else mentioned, a massive air pocket will wreak havoc. Years ago, I had a 455 Trans Am that was notorious for air pockets after a cooling system drain. I eventually tired of this and whipped up a MacGyver-looking vacuum bottle contraption that did a nice job of pulling the air out of the system. Much torment was avoided from then on.