Or consider this... What makes a 600 a 600? For some it is the power and for others it is the interior. It is not really quite that simple, but if you don't want to go into insane detail you could far more easily swap out the interior of a 600 into a lower model as long as they had the same wheelbase.

Both would be big jobs and would depend on what you had to work with, but the interior swap could be done over the course of days and the engine swap would be something that would tie up the car for the length of the swap which would involve matching up the electronics, the exhaust, motor mounts, transmission linkage, drive shaft length.... Did I mention I have tried these sort of swaps before?