A swedish carmag had a chat with a guy from BMW and this guy said the engine fits the enginebay in the new 3-series. A 350d is going to be a rocket if it becomes a reality.
Is it known how much heavier the 50d engine is over the 35d, it might be the deciding factor as to whether it makes it into the 3 series.
There is a few 550d for sale in Sweden and they cost as much as standard M5. But then again , these 550d is loaded with extra options.