I'm trying to understand what the heck is going on too.

It sounds like the new pump bolts up to the engine but the pulley does not fit - and it does not line up with the existing "lines" in which the belts have to run.

First step =>

Compare the "heights" of the water pumps - does the old one stick out further from the block?

There are different pulley options out there - you could be suffering from a bit of a swap around

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!