If I'm not mistaken one of the main problems with finding a transmission that will work with these cars is the shifting mechanics. I believe that there weren't many models that used a combination of just vacuum and a kick-down switch without the kick-down cable like the 80-81 300d's do. I have an 81 myself and was worried when the transmission died a few years ago. The solution wound up being an 84 240d transmission. It had a kick-down cable so we had to rig a way to attach it to the throttle but it has made a tremendous difference in drivability compared to the old transmission. As far as I know any of the transmissions that will bolt to the engine will work except for the turbo 300d transmissions which from what I hear are geared differently. Sorry I don't know exact years but I would assume at least 79-85 240D's and of course the 80-81 300D's will work. I think that some of the earlier 300D transmissions may work but I really don't know much about them.

I think that changing both the engine and transmission from a turbo 300d to your car will work without welds. From the junk turbos I have access to there are no visible difference in the mount points and I believe the chassis is identical between the two type of engines. I believe the only difference is the rear end gear ratio which on the non-turbo's is a bit lower.