You're wanting to replace a 420 with a 380? Its not a wise decision... especially if its a US spec 380 engine you're putting in. The 380 US spec was probably the worst mercedes v8. Not only did it have very little power, if its an early 380.. it had a single row timing chain that would snap. A 420 engine should be as easy to find and not cost any different than a 380. They are however both m116 engines and should swap right in... but not a good idea. Just my 2 cents.