Thanks for the pics Greg. That confirms what I've seen in other places. I've never seen a spacer on the std ratio set but I haven't done many 904's so it's possible some may have it, but I doubt it. Man those teeth don't look good, bummer!
I think I just realized what the spacer is for - it's because of the step at the bottom of the sun shell. The thrust washers are all different too and I believe there are more of them. The overall height of the gear/shell assembly is the same between the two but the internal parts are somewhat different. That's why you need to the whole shell when you get a low gear set because it won't work without that little spacer! That's the whole thing right there! Not 100% sure on that just a guess but it makes sense.
I'm not sure what the output shafts from the later transmissions look like but I am guessing they are the same since you can swap the low gears into an older trans without getting the later output shaft.
And yeah, unfortunately that gear set is toast. The carrier was really stuck in the ring gear, I had to smack it a few times with a dead blow to get it to roll and drop out.
I believe the trans that is currently in my car has a low gear set in it. It would have provided some extra torque multiplication with the 3.31 stroke 340 but it won't be necessary behind the 416 I am building. I may end actually up leaving that trans alone for a the time being but if/when it comes out for a rebuild, it won't have the low gear.