I had a good look at doing this a few years ago & its not as complicated as that interesting 560 transplant. I was, however, considering a a 450 KJet retro fit into a 380 KJet ,- a bit easier as in your case the FI would have to be done too. Or perhaps a conversion to megasquirt as well - see Bari's upgrade at:

http://www.mercdjetmegasquirt.britautorepair.com/

You would really need to have a whole 450 SL donor car, not just a 450 engine, as all the extra stuff to get would make it a marathon conversion (for example an early 450SL 'DJet' air cleaner is needed).

The diff could stay - but it would be important to swap the front springs as the 450 is a lot heavier than an all alloy 380/560 motor.

I would say it might be worth doing if you had a good 380 SL body. In my case the SLC body was much worse than I originally thought and I was glad I didnt go ahead.

Good luck with it - Peter in Melbourne (very cold!!)