I would agree about researching beforehand but the M20 engine and gearbox in my Cabrio were from an E34 525i and I didn't encounter any significant problems (bearing in mind that this powertrain option was never originally intended for the Cabrio !).
As people have already said, I think that the engine and gearbox are fundamentally the same between the BMW 3 and 5 series.
I'm not sure what the ratios are in the 3 series but the E34 525i ratios are as follows
1st = 3.83
2nd = 2.2
3rd = 1.4
4th = 1.0
5th = 0.81
Final drive = 3.73
I assume you'll be using the BMW final drive, but the above ratios worked very well with the Sierra 3.92 diff in my Cabrio as you don't need the long gearing of the BMW (100mph is scary enough let alone 135mph !).
The reason I went this route was that it was far easier and cheaper to find a 5 series donor than 3 series.
Hope this helps!
Simon
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