You K20 could have a 14SF of 14FF.
If it is a 14SF G80, the detroit is a complete carrier replacement, so you have to pull the ring of the old diff set up the gears again.
If it is a 14FF G80, I'm not sure if the G80 unit can be pulled out of the OE carrier. If it can, the detroit is a drop in. If not, you need to source an open carrier to drop teh detroit into, and swap carriers like above.