I use liquid soap on the blades of the impeller. With one hand on the cover applying moderate pressure, take the other hand and rotate the drive shaft CW while the cover sinks down and mates with the bottom plate.
The copper output tube has seals on both ends sometimes and other times the seals are not mounted on the tube...if you didn't have any when you took it apart don't worry about it. You do need to ensure that the copper tube mounts in the receptacle in the bottom of the engine block. Some times it is easier to mount the tube in the block and then into the water pump as it's easier to see that way.
