Replace 617 with 616
 

Maybe replacing 617 with 616 in 77 300D Auto. Will the 616 bolt right up? or do supports have to move, linkages and driveshafts lengthen, etc.

I'm pretty sure that changes will have to be made to the drive shaft. Though, if you can source out the front half of a 240d drive shaft, and are using a 240d transmission, it may all bolt right up.

If you are swapping in just the engine I think you'll need the front piece of a 240d drive shaft.

So if the back end of the engine comes forward, then the tranny support moves, linkages stretch, driveshaft too. That makes it a project- not so easy then.

What's wrong with the 617, is it terminal?

Should still be easy, as long as you find the linkages/driveshaft/support from a 240d.

usually people go the other way around

You may need a cross member as well. To get correct 240d performance you will want to change the diff and the speedo as well.

I've darkened a suspect phrase in your post.....I don't believe it is accurate. :D :D Those two words do not belong next to each other. ;)

Running a 346 diff with an automatic 240d would change the acceleration from really really slow to glacial. Once up to speed I believe it probably would cruise at whatever speed you like and slightly beat a 240d with stock ratio on mpg. Crossing busy intersections and merging ramps would be an exciting adventure though.

You could make custom engine mount arms for the 616 and keep the driveline stock. This would have the added benefit of moving the engine back a few inches (better handling).

That would be a lot of work, more than altering the driveshaft. It would also put the engine back from the radiator and possibly pay havoc with the power steering and ac hoses.

I think not recommended.

A good thought though.;)

(The handling of a stock 123 240 is very balanced to begin with too, IMHO)

I did this swap a few years back, it helps to have the donor car around when you do it. front driveshaft, crossmember and linkages were some of the problems I ran into but I did it in my parents driveway when I was just a young MB enthusiast :) Two brothers who were friends of mine had the cars, the 300D threw a rod and the 240D hit a telephone pole. 240 engine into the 300 body and one of them was back on the road again :)