Conversions, especially ones where there isn't prior development work already done for you, can be very costly and problematic. I don't know what the weight of the original engine is, but you would want to make sure that any replacement isn't significantly heavier or the handling and braking can be severely compromised.
If you really must do this, by all means I would stay away from the
2.9/3.1/3.4l family of engines. I don't know if they were ever made for rear wheel drive or not, so it might be a moot point. Packaging efficiency is very good, but problems like leaking intake manifold gaskets and other make for a less than ultra-long-life design.The 3800 from a later model Camaro or Firebird donor would be the first thing I would look at if I just had to make the conversion. Other possibilities might be the V-6 Mustang motor and transmission from 1990s vintage stangs. Lots of fabrication and making do would be involved in such a swap.
Lots of folks have done GM swaps into Volvos, Jaguars and Jeeps, but I haven't seen much written about doing so into an MB product.
Personally I would tear down the original MB motor and make it right. The cost and headache will probably be lower in the end than using low cost GM parts as the basis for a swap.
John