38-39 was the last of the spiral cut gears. Starting in 40 they went to conventional gears (can't remember what they are called, hypoid?) The gasket pattern is indeed the same but there are differences. 38-39 had the ring gear rivited to the carrier. After that, they were bolted on. This shifted everything by 1/2". The change is reflected in the axles. 38-39 axles are the same length. After that, the axles were of different length. They also used different splines on the axles. In addition, axle housing shifts everything by 1/2" to compensate.
You could use the complete differential and housing from a 40 plus, except, the propeller shaft, torgue tube will probably be a different length. Check the parts book and you will see the the propeller shaft came in a bunch of different lengths. You may get luckey and find one the same length (possibly 40) but probably not.
