changing an engine/transmission even when it came from the same chassis is a huge project because almost everything else in the car was built around that specific engine/trans combo.

At one time I was thinking about installing a M117 from a 560SEL into my 300SE making it a 560SE (In europe swb 560 was available) and the list of things I would have to change ended up being so long that I decided to scrap the idea altogether