interchangeable parts:
diff gears (although they won't necessarily be the same ratio...depends what trannys they have).
tranny (although, if both are 5 speeds, the one behind the 16V will have a .795 5th gear, where the one behind the 8V has a .865 5th gear).
clutches/flywheels (the 16V fiber plate is bigger diameter, but will fit on an 8V flywheel and vice-versa). The pressure plate part of the clutch, and the flywheels, are the same between 8Vs and 16Vs.
tcase.
axles.
there are other swappable parts....but the ones listed are the major ones.