I've got an original set of service manuals for the W123's and although I cannot tell if you can switch driveshafts, I would have to say your chances are good! According to my books, both the 300D and the 300TD both have a wheel base of 2795cm or 110 inches. The only thing I can think of is with the wagon, you could have a difference in centre shaft support, but you would have to remove it and lay them out side by side. Hope this helps.