999 clutch pack in a 904' & opinions on fluid

Not so much with the 42/44 stuff as that will all interchange with each other, although I do believe the 42 may have had 3 pinion planetaries front and rear, so no reason to go from 4 pinion to 3.

The 1-3 portion of the 42/44 trans is all very similar to the 999 but the majority of the parts will not interchange with a 904, 998 or 999. The later model 4 speed used a bigger high gear drum with two bushings, a larger diameter sun shell, and specific input and pump. The few parts that will interchange with a 904/998/999 are the front and rear planetary, ring gears and sun gear.

To my knowledge and based on what I've torn down in the past, the 998/999 all had the 2.74/1.54/1.00 four pinion steel planetary gear train, and the same holds true for the 44.