You can't swap autos between engines because the ECU and TCU know which one of each they should be talking to. Plus the 3.7 has a 5hp19 and the 4.2 has a 5hp24.
My solution would be to manual swap an S8 with a TDI gearbox. You get the same 1-5 as the auto, plus a super-high cruising gear. This is the result:
