There were a great many other modifications done to the W124 chassis to fit the M119 V8's. Most of those modifications are not practically retrofittable. That is why you don't see W124 cabrio V8's as well... The amount of headache and funds needed to do the V8 (if even possible) would outweigh trading up to a newer/more powerful car several times over. You could try a supercharger or twin-turbo kit for the I6 though. You can also bore/stroke the M103 up to 3.6 liters for some more power. If your current engine is getting a bit "tired" you may want to consider a performance rebuild to 3.6 liters by one of the tuners. Fill out a Brabus inquiry and see what they might have available for you.

Hope this helps...Lee