I performed a conversion on my '82 300D, because the car's engine was worn out and I couldn't afford a new one... I installed an astrovan 4.3L V6 and T700R4 transmission in it, and have been extremely satisfied with the results. I'm not sure it would be a wise conversion for the much more desireable 380SL, though. It seems to me that you might want to keep the car pretty original, and I've seen several engines from time to time that would fit your vehicle at reasonable prices. If you are sure that you would like to do the conversion, however, I'd be happy to post the info from my conversion for you. I had it up on a web page at one time, and could reinstate it for you...
My conversion cost around $900, and gave me a very well balanced vehicle, that is fun to drive, and that has been completely trouble-free since the conversion almost 2 yrs ago. It gives me around 28 mpg on the freeway at speeds of 75 - 85, with 4 people in the car and the AC on, and around 20 mpg in town. The T700R4 is an excellent transmission for the car, but it's about 10 inches wider than the Merc tranny was, so I had to widen the tunnel by 10 inches. I accomplished this by jacking it apart - it only needs the width at the very bottom. I was able to use the stock mercedes engine mounts, and made some adapters to fit the chev engine mounting points, which with the V6 sit nicely centered right over the Merc motor mounts. I had to shorten the front of the pan by 2 inches, but since then have noticed that the pan used on Blazers with 4WD would most likely fit with no alterations. You'd need to also use the Blazer oil pickup. I use my car almost every day, and have driven it on several long distance trips, and it continues to provide much satisfaction - nice, very quick car with loads of comfort.