Without removing everything you are pretty much guessing as to what the problem is. That being said, the most common reason for leakage after a water pump change is a rolled o ring when sliding the water pump back into the housing. I wouldn't think the shaft seals on a new water pump would leak but stranger things have happened. It never hurts to do a little squirt with wd40. You just have to make sure all o rings are seated correctly and all surfaces are clean. I have only done a couple of water pump changes, one on my bike and a couple of friends. I can see it being rather easy to roll the big o ring when sliding the pump into the housing. Let us know what you find.