Waterpump housing

Does anyone if there are 2 different water pumps for big blocks. I have a 76 passenger side exit waterpump housing and a 68 drivers side waterpump housing I assumed the water pump would be interchangeable but when I tried it doesn't fit correctly. Are there 2 different size pumps or maybe there is something wrong with one of the housings?

the water pumps are the same, the only diff is the sometimes the impellers will have a different amout of blades. non-air or air

I thought they were all the same, any idea why mine won't fit? I can't see any imperfections etc. Does the waterpump have to be bolted in with the word "top" located at the top of the pump seemed like it might fit better upside down.