I did that conversion last summer and just got it all to work right about two weeks ago. The water pump was off a 535i (e28) I don't have the part # in front of me. The biggest hurdle that I had with this conversion (besides the usual water pump hell) was the newer style water pump has a different bolt pattern on the pump face than the older style. Thus it requires a new pulley, or some precision drilling on the old one to make it able to bolt onto the newer style pump.

The pulley is "hub centric" and if the drilling is not done right it will cause the whole assembly to wobble and shorten the life of your water pump, and make you feel foolish when someone looks under your hood when the car is running (AMIK).

I finally decided that the pulley would need to be replaced to make this whole thing work correctly. I purchased a pulley that would fit the later style water pump from Al Taylor, and when I got it looked at it with dismay because it was only a single row pulley and not a double row that was on the car before. As I learned and everyone else probably knows already that BMW changed to a single row pulley sometime between when the E3's went out of production and this newer style water pump went into use. I think this happened with the E12's or the E24's but I'm not sure, I'm sure someone out there with more BMW knowledge would know. Ok the pulley fit great, but the only problem was how to run the power steering pump without a second row on the pulley. I ended up using the pulley on the front of the engine that the AC compressor used to run off of. The power steering pump and the front of the engine pulley line up perfectly and there is no binding or rubbing of the belt. From some recent junk yard trips I have learned that this is also how the later BMW's with the M30 engine had their power steering hooked up. It took about 5 trips the the auto parts store to find a belt that was the right size, but once that was done everything came together nicely. If I ever decide to put AC back into my car, I'm going to get a crank shaft pulley from a later M30 engine, because it has two rows instead of one (like on the E3). One for the power steering and one for the AC (Everyone probably knows that already too).

Hope this helps, and isn't too long winded