I agree, a rattle is likely to be from something outside of the transmission. If you can get it up in the air so you can check out the box and OD + the driveshaft and centre bearing, before you start tearing it down, you might find something. Loose gearbox cross member or speedo cable hitting the exhaust might be worth checking. Depending on how hard the car was hit, that might compromise your investigations. Did you post a pic of the crunched rear end some time back?
Yes the D type OD is the early one and the J the later version. The change over is approx the change from B18 to B20 but I've not seen that in print.