Something to think about is the rear-end ratio. At least for me the 3.07 rear-end ('84 300d) does too many revs on the highway. I think the 2.2 has a 3.42 ratio, meaning about 3k rpm at 60mph. Just my opinion that the 617 won't really be efficient or long lived if you do lots of highway driving. It will be underloaded especially since the 190 weighs some 700 lbs less, and regular underloading is not good for diesels. If the 190d has overdrive or fast acceleration up to highway speeds is the goal then thats another story. I don't think swapping rear-ends is any easier swapping than engines between the two cars.