Here’s a post I started late one night this summer and never finished. The long story short is you can get very impressive performance from an BMW e30 by stuffing a 3.5L ‘big six’ under the hood! Here’s some links:
Jared Squires BMW e30 M30 Conversion Details. Considered the best source for M30 swap information.
Step by Step 335i e30 M30 Construction Details at e30.de
Step by Step e30 M30 335i Build Up Post at R3vlimited.com