Basically the best way to do it I've seen is to drop the front suspension, motor carriage, transmission all out the bottom as one unit. Do the same to a donor junker, fix all that can be fixed while it's out, heads, clutch, linkages, drive line, manifolds, do anything and everything you can before putting it all back in 'cause it stupid easy out of the car. Then replace the engine, cradle, front suspension, transmission as one unit. New ECU, tighten everything down and have fun. There were some posts in here I remember from some people that did just that, only in an E30 (the earlier 3 series). Do some browsing we're not breaking any new ground here I'd swear I saw the step by step somewhere.....