I just got a 81 diesel 1L and 2wd tranny. The bell housing on the diesel appears to be the same bolt pattern but the diameter is an inch or two larger so a straight swap is not possible compared along side a 82-83 4wd transmission the transfer case will bolt right in behind the shifter. The cover on top also has to be swapped.Length of transmission is the same from mount to front edge of bell housing.
The gearing should be the same. The extra fuel milage is due to the diesel motors efficiency

 

The little diesel swap will be fun, I still haven't got to start on mine yet, but since the world didn't end yet there may still be time, ha ha.