I also had a '84 190E 2.3 five-speed. it had a lighter duty five-speed than the later one that appeared on the 300E, and other six cylinder models, and I think it was also used on later W201 four-cylinders.

The '84 has slightly different ratios, like a 3.91 first vs. 3.86 in the 300E five speed. I don't know what, if any difference there is in physical dimensions.

Also I think your '84 has a different, "lighter duty" axle design than the later four and six cylinder models that may create some integration issues.

It's probably doable, but you'll likely have to deal with some issues. Since you have both cars now get them up on lifts and start measuring things.

Also keep in mind that the lighter duty '84 driveline components may not be as durable on a six cylinder, especially if it is run hard.

Duke