An e34 bay will take almost anything. A V8 diesel truck engine, from some GM shouldn't be too hard- I've seen a few older GM V8s in e34s, perhaps even LS1s have been done by now. There are many diesel ones... get twin turbos if you want economy and you'll be laughing. Feinately need a 540i driveline though- the big diff and rear end hardware will be a must.