I have just gone and bought an E36 325 Manifold for my E39 528 and still trying to gather all the other parts I need to peice it together. There is a good work instruction at this web site http://www.team-bar.com/diy_m50manifold.htm
The results by all accounts look very promising, as long as you have given your car a very good service first. No point trying to up the Bhp if you have injectors that have deterierated to knock your total horsepower. I think the problem with doing this on a 5 series like mine is the air box is very restricted, anyone got any ideas how to improve total air flow in such a confined space?