There is nothing on it in the tech section because it's pointless. For all the effort you would go to to do that you might as well actually put a 2.8 in it.
The 2.8 and 2.9 share most of a short block. The similarities pretty much end there. The top ends are just too different to directly swap a 2.8 carb system on. The ignition controls are dependent on the computer. The cam's are different. The dis drive gears go in opposite directions, so those can't be swapped back and forth directly. The 2.9 has a tri-runner lower intake that is setup for ported FI, not a carb.
What exactly is your reason for wanting to do this?