Will a transmission out of a 1983 300SD fit and work in a 1979 300SD. I know that the 126 has an electronic speedometer and maybe the tailshaft is different. Can this be swapped between transmissions? What are your thoughts?

Well, these are two totally different transmissions. I have never measured the lengths, so I don't know if the mounts would be the same, or the driveshafts.

I seriously doubt that the speedo drive would swap over as I am sure the back ends of the trannys containing the speedo drive gear would be different. You can swap the back end from a 722.315 (300D turbo) on to a 722.303 (W126 300SD) if you want. So you are going to have some issues- anything can be done, but it won't be straightforward.

Rick

__________________
80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??)