904 low gear- which vehicles and which internal parts?

The problem with trying to build a low gear 904 is there were numerous versions of 904/998/999 transmissions with minor differences in clutch pack carriers, pumps, etc. that won't mate up properly so it's hard to find info that covers all the bases. Your best bet is when you find a trans with the low gear set (easily identified by the sun gear having different diameter splines on each side vs. same splines on both sides of a std. ratio) and just rebuild it.