I've seen plenty of broken M51 bmw engines. Enough to know that there are two different pump flange sizes. A 68mm and a 50mm. Check the engines on BMW oem for the 325 or 525's if you dont believe me. Don't know why ye haven't come across yet (although op did) Also I swapped many 325's into...

There are two different size pumps.One has a 68mm flange , the other has a smaller flange. Look at the engine where the pump fit into, does it look like its removeable,if so, take it out and modify the hole to make it bigger to accept your pump. Some engines have a removeable support (btw check...