Several years ago I bought a 1954 Chevrolet 4400 with a remanufactured 1958 235. As with the previous owner I struggled with continuous overheating that would not go away. I changed the thermostat and the radiator and then I added electric fans thinking it wasn't getting enough air. It still did not cure the overheating so last weekend I pulled the water pump out thinking it was not circulating. Look at the pictures below and you can see that I found a water passage that is in both the head the block and in the water pump that was plugged solid. Without this passage being open the water cannot flow from the back of the motor to the front of the motor and get the thermostat hot enough to open before boiling. I hope others can use this information and save them some needless expense.
I'm amazed anyone would claim an engine "rebuilt" if they didn't bother hot tanking the bare block to remove all the accumulated rust sediment . I often find 3" or more of silt around the cylinders when I remove the welsh plugs, it's not terribly hard to dig it all out with coat hangers or welding rods... At the very least I'd remove the block drain plug/valve and ensure coolant flows freely out of the hole .
