I used the J case because they are the same structure and it meant less time converting the output shaft. The planetary gearset was the same (both 3) - which i was very disappointed in - so i basically just converted to the wider chain along with larger shift fork and shift collar from the D case.
So if nothing else i got the HD chain (which runs as a kit for 425) for 150. and spare case halves and spare chain/gearsets.
the hardest part about everything was the snap rings. even with snap ring pliers they were a ***** and took forever.
all i have left to do is get a new tailcone (mine broke because of a blown u joint - i think) or try and seal mine up and put that on, bolt the case into the jeep, fill'er up with oil and go do some wheelin!