I've done that swap on my E30 Touring
The following pages detail the installation of the M30 3.5L engine into the e30 chassis.
The designation "M30" refers to BMW's "Big Six" engines, those that were installed in the 5-, 6- and 7-Series. True to the motto, "Nothing can make up for displacement ..." the "Big Six" is put into the small 3-Series. The result is an amazingly powerful driving experience
http://www.e30.de/335i/00.htm