Yeah, the splines where the planetaries sit,on the 998/999 are different, so the entire geartrain has to be swapped as a set including the output shaft. And that means you would lose the rear pump. Furthermore, I have no idea if the case can be made to work like that.
The ratios of the A999 are
2.74-1.54-1.00 with "wide" splits of .562-.649. The A904 is
2.45-1.45-1.00 with "close" splits of .592-.69
Not all engines are happy about that. It doesn't look like much on paper, but with a stock lo-stall TC the engine is fine in first gear, but second, if shifted too soon, bogs down.
I put a 2800TC on mine and then it was pretty good, and I could run the next lower rear gear for hiway fuel-economy.
